Brighton or Cornwall?

Brighton makes far more sense geographically, as you would have to leave Cornwall as soon as you got there. Brighton beach is pebbles not sand, but it’s an easy day trip from London. My favourite English beaches are in north Norfolk and Northumberland, neither of which are possible in your timescale.

Favorite seaside town for us would definitely be in Cornwall, not Brighton.

You do not have to go all the way to Lands' End to experience Cornwall. We had a lovely 2-night visit at Port Issac, accessed by train to Bodmin Parkway and taxi from there.

If you are going from Oxford, the train to Bodmin Parkway takes 4.5 hours, with one change on the way. For comparison, the train from Oxford to Brighton takes 3 hours, with 2-3 changes on the way.

If your entire trip is just one week, I would strongly advise you to scale back a bit. Giving yourselves just 2 days to see London, including recovering from jetlag, is pretty minimal. You've already said that Oxford is a high priority for you. Is it also crucial for you to visit a seaside town? If so, I would say to save the Cotswolds for a future trip.

I know it can be frustrating to travel all the way across the Atlantic and not see everything you'd like to see, but with the itinerary you've outlined you will be exhausted rushing from place to place and you'll be experiencing the interior of trains and/or a rental car rather than experiencing England.

If you're really set on Cornwall, there is a sleeper train that would save you a night in a hotel. Choose one town that is on the rail route and make that your Cornwall experience so you won't have to rent a car.
https://www.seat61.com/Cornwall-sleeper.htm

Read descriptions of the towns and decide which one appeals to you the most. See, for example,
https://www.lonelyplanet.com/england/devon-cornwall-1341739

Frankly I wouldn't bother with Brighton, but YMMV.

Also, be aware that the south coast of England can be cold and rainy, even in the middle of the summer. The Cornwall weather you see on the Doc Martin TV show is not typical. You might be lucky and have a nice warm, dry day, but that is by no means guaranteed.

I know you're excited and trying to see as much as possible, but I'd seriously scale this back. If you only have roughly a week on the ground, then I'd use London as a base with one day trip to Oxford, since you mentioned that is a priority. I'd certainly skip the Cotswolds which will require a car. If you want a second day trip, then Brighton is easy enough, although keep an eye on the weather. I can't imagine Brighton is much fun in a cold rain.

What time of year is your trip?

I agree with Dale that two days for London on a first trip (and I fear you're counting your jetlagged arrival day) is way beyond inadequate. Now, if you really dislike big cities, there's nothing wrong with skipping London entirely, but I wouldn't head into the city for just two nights.

Brighton has a funky charm, and I enjoyed the Pavilion and the local museum, but it wouldn't be a top recomendation for me. I did very much like the day-trips I took to Lewes and Arundel. Neither is on the coast, though.

On a short trip like this, I think it's really important not to try to cover too much territory, so much as I liked Cornwall, I would not try to squeeze it in. The travel time is just too long.

Oxford, on the other hand, fits well with London. Better still, there is at least one company that does van trips to the Cotswolds from Moreton-in-Marsh, which is a short train ride from Oxford. You could spend 3 nights in Oxford, allowing time to cover that city and take a one-day tour of the Cotswolds. You could also rent a car for the day in order to tour the Cotswolds by yourselves.

I really appreciate everyone’s advice. We're planning our trip for the last week of August to the first few days of September. We are also going to visit Paris for a few days after England. I agree our itinerary is too tight and we need to scale back. I think it’s important to narrow it down to what will mean the most.

London has so many exciting things we both want to see. The British Museum, Buckingham palace, and Westminster Abbey. But I’m not the biggest city fan in the world so we’re thinking just a couple days there. I’m a huge C.S. Lewis fan so in Oxford I’m excited to geek out in the Eagle & Child and maybe head out to the Kilns and Holy Trinity church.

And ever since I was a little girl I’ve wanted to see a slice of England that looks nothing like home. Quaint villages, rolling green hills and stone ruins… things I wouldn’t see at home on the West Coast of the USA. Part of the fun for us would probably be renting a car. Oh and did I mention, CASTLES?!!!! We don’t have anything like that where I live. At least one epic Castle is a must.

Maybe my thinking on the Cotswold’s is all wrong…but then I see pictures of Bibury and I’m like… wow that looks amazing.

Anywhere else that has all of these things in one? I know, I want the moon and the stars ;) But with a place like England who can blame me. Thoughts?

If you enjoy castles, you may enjoy Leeds, Bodium, or Dover. Near Leeds castle is Igtham mote, an old manor house surrounded by a mote. They are all in SE England. Cornwall is a distance away, and driving is slow there because of the roads. Unless you had plenty of time, I’d save it for another trip. There are plenty of small Cotswold towns to see Chipping Campden, Bibury, Bourton on Water, Stow on Wold, Stratford Upon Avon, Tewksbury, Tetbury, Circencester, Broadway, etc. Most of the Cotswolds towns and villages can be seen in a relatively short amount of time, so with a rental car you can see a few each day without too much problem. To me, other than the pier, Brighton has little else worthy of visiting.

You can visit two very different castles as a day trip from Oxford. Kenilworth ( ruined, somewhat restored) is sixty miles away. Warwick is only a few miles from Kenilworth - it's completely restored and very busy - although touristy, it also gives a feeling for what castles must have been like when they were actually in use and humming with constant activity. Both are large castles and were once quite important in English history.

If you only plan one day, you will not have enough time for Cornwall.

On my 6th visit to England, I finally went to Brighton in June only because my daughter wanted to go. The aquarium was really fun but Brighton wouldn't be anywhere on my top 25 list of places to see in England. On the other hand-London I never get enough of. Tower of London is the easy castle to do. Dover is my favorite and could be done as a day trip from London. I've been on bus tours to the Cotswolds twice and find them to be cute but very boring-I think they would be a waste of time on a short visit unless you really want to hike. If you insist on the Cotswolds, I think London Walks has a day trip there from London, as does Evans Evans bus tours-that would be enough for you to at least get to see the quaint villages(and this is heresy from someone who's only seen them by bus tour, but in my opinion, if you've seen one, you've seen them all.). People on this forum have also mentioned Secret Cotswolds tours.

I would skip Brighton, it doesn't really have much to offer. OK for a day trip for nearby residents but I wouldn't advocate it for foreign tourists who have little enough time as it is.

Beachy Head I presume for The Seven Sisters?

Cornwall is simply too far and travelling there will eat up your time. Whilst it's a beautiful part of the country there are many other parts that are just as attractive.

You mentioned a seaside town. Do you want a town simply by the seaside or more of a Victorian resort feel?

I think with your requests for castles, picturesque villages and a seaside location then the Isle of Wight would be a perfect match. There are some fantastic seaside towns, Shanklin being my favourite and one which is very picturesque, full of thatched cottages and windy roads https://en.wikipedia.org/wiki/Shanklin. It also has a very good beach and is a traditional English seaside resort.

There are also several castles, Carisbrooke http://www.english-heritage.org.uk/visit/places/carisbrooke-castle and Yarmouth http://www.english-heritage.org.uk/visit/places/yarmouth-castle/. There's also Osbourne House, Queen Victoria's holiday home http://www.english-heritage.org.uk/visit/places/osborne.

And to fulfill your cliff requirements there are The Needles http://www.theneedles.co.uk.

The island can be reached easily via ferry from Portsmouth or Southampton (car or foot passenger) or via hovercraft from Portsmouth. Both have direct rail links to London so you could have your few days in London coupled with a quick side trip to Oxford and then travel down by train and take the ferry across to the island. Personally I would rent a car there but it can be explored using public transport.

Isle of Wight is a very good idea, i second that.

I echo the comments to scale down your expectations. You will regret not spending more time in London. Yes, London is a big city, but it is not a typical big city. There is so much to see and experience. Tower of London, Westminster Abbey, Buckingham Palace, St. Paul's Cathedral, British Museum, National Gallery, Churchill Museum, British Library, Victoria and Albert Museum, Trafalgar Square, Hyde Park, Shakespeare's Globe, etc., etc. London doesn't feel like most biggest cities in the U.S.

And, don't forget that you will have to deal with jet lag.

I also suggest you skip both Brighton and Cornwall. It's not worth the time or effort for your quick stay in England. Focus on London, Oxford, and the Cotswolds. If you add anything to the itinerary, it should be in this corridor - so, may be Kenilworth or Warwick castle. Consider also Bath or Stratford-upon-Avon. (but, no way to fit them all!)

I'd spend a minimum three days in London, which likely means 4 nights. Something like this:

London - 3-4 nights
Oxford/Cotswolds - 2-3 nights (not sure exactly how many nights you will have); use one as a base to visit the other. I'd strongly consider to rent a car for this part of your itinerary. This gives you more flexibility and it is so much easier to do the Cotswolds by car. Consider also to rent a bike for part of your Cotswolds tour. And, note that driving in England is a fantastic and memorable experience. While there is a learning curve, it's not as difficult as you may think. If you don't think you or your husband can handle it, train or bus to Oxford and then to Moreton-in-Marsh and do the guided tour suggested above.

I know the Cotswolds are a favourite on this forum (and others) however I feel that they're popular because people go there and only there (or on the tried and tested route of London-Stonehenge-Cotswolds-Bath), they rave about it and consequently others choose to follow yet there are swathes and swathes of similar places throughout England but because they don't feature on the radar of many tourists they remain unnoticed and what you end up with are constant recommendations for the Cotswolds to the point of saturation. Many visitors often complain about the number of tourists, buses etc yet there are hundreds upon hundreds of villages up and down the country that are free of such an invasion.

In light of this it was the basis for my recommendation for the Isle of Wight as it also features many of the other criteria that the OP listed. Seaside, castles, cliffs etc. The Cotswolds, whilst pretty, has none of this and is the main reason why I would not recommend it to the OP. With only a short amount of time in the UK to fit in London and all the other interests it makes sense to focus on a relatively compact area that offers everything and the IOW fits the bill (IMO).

I'm not a Cotswolds naysayer by any stretch but I feel compelled to shine the light on other areas of the UK that are often overlooked by overseas visitors simply because they're not aware of them