How do we travel all over the UK in 16 days?

I have never heard about restrictions taking a car in or out of Ireland. Why else would they have car ferries? I see Avis has rental locations in Bristol, they will reimburse taxi cost between Temple Meads station and their location 1/2 mile away. Return it there and you are 2 hours from Heathrow (Bristol-Paddington, then Paddington-Heathrow). You may want to contact Avis direct about travel restrictions.

I am also reading that the Cork-Swansea Ferry went out of business last month. Best bet is Fishguard-Rosslare.

Thanks for the info...I didn't see that about the ferry. I'll see what Avis says. We have read it was difficult to take a care out of one country. I suppose it's more difficult for them to get it back to their country. Trying to plan the trip is very time consuming, and would like to learn as much as possible. We will carry a backpack and small suitcase, enough to wash every 5 days...just what Rick recommends. Again, any and all suggestions would be great!
Thanks again, Ann

If you call Avis or anybody else in the US you'll get bum scoop. Call somebody in the UK or the RoI and you'll find there's no way you're going to take a rental car across the Irish Sea and not be in a heap of trouble. Break it and you bought it - - and still have to get it home. The ferry operators don't care and won't check, probably. As to why they exist - - probably to haul privately owned cars, food trucks, etc - - kind of makes sense. You Wales and initial England scheme is a bit vague, but you can drop the car at Holyhead and get another one on the other side. These are separate transactions throught the car rental companies, the ferry operators aren't involved. From NI, I'd probably fly to Scotland and start over - - car rental joints are kind of slim on the Scotland end of the ferry routes.

That is what we read about cars in Ireland. I think flying from Belfast to Scotland may work, and then take a train to Edinburgh.
As far as the vagueness of London, we will stay there for 2-3 days without a car and take a bus or train to bath area. Then find a car and take it to the ferry and drop it off before we get to Ireland. Possibly rent the car through Ireland and Northern Ireland and leave it in Belfast? I will look at a couple of places to rent cars and see what they say? Thanks

Can you take the hire car across, yes, should you, are you feeling lucky
Have you considered flying into Dublin, hire the car, return to Dublin, fly to Edinburgh, then hiring another car returning at a number of venues just out of London. Airfares are cheap, 70€ Dublin to Edinburgh. Read the Ireland and English car rental co website info, and each country,s sites are set up differently especially for insurance. I recall their prior permission is required, I went no further.

wow! About a 40 day geograpically speaking trip in 16 days. All you will be doing is driving, not seeing anything. You could get to all those places in 16 days, but that is all you would be able say, I was there, did't do a thing.
Best to stick with England proper, maybe a short run to Scotland at the most. Good Luck!

Be advised that if you intend to take a rental car into Ireland, let your rental car company know when you rent the car. You will most likely need full CDW when driving a rental car in either the Republic of Ireland or Northern Ireland. Usually when you rent a car, you waive the CDW as your major credit card has features to cover you. Not so in either Ireland for Visa. I have a copy of their web page about it. I think Mastercard may be the same. Secondly, we rented our car on arrival at Heathrow and spent 3 weeks in England. No Wales, Ireland or Scotland. With one of the weeks in London, we drove and stayed in Bath, Cotswold, Lake Country, York and Cantebury. In each location, our day trips were to major castles, churches, gardens and beautiful villages (i.e. Castle Coombe and Lacock). We also visited Oxford and Cambridge. With 2-3 nights in each location, we easily used up the 2 weeks before turning in the car and touring London for a week.

After reading these posts, we are doing some rearranging. We still think we can make this in 16 days seeing everything we've planned. Except will take the ferry over to Ireland, rent a car and explore for about 5 days. Return the car in Belfast and fly to Newcastle where our daughter is living at the time. We will then rent a car and explore through Scotland for a few days...get the car back to Newcastle and fly to London. It seems to be worth not wasting an entire day taking a long train ride. Just driving and seeing the beautiful country is an experience in itself. We will Take the trains and buses in London and vicinity, rent a car from Bath to West Wales...take a ferry to Ireland, drive another car through Ireland for 5 days ending in Belfast. Fly over to Newcastle, spend another 4 days and then leave on a 9am flight out of Newcastle in order to get our flight out of London around 1pm. So far so good... Now all we need is to find little B&B's along the way....It's an adventure...I think we should be fine during the week just finding a place to stay, but would like to have reservations in Dublin, Belfast and possibly Glasgow, I'm pretty sure we will hit those cities on the weekends. Our family consists of 2 college students, Mom and Dad...We have traveled all over the United States and out of the country, so travelling is very natural to us. I am familiar with driving on the left side, so that is not an issue. We will be caring small carry-on suitcases with our backpacks in order to get any place necessary easily... Again, I appreciate all the suggestions!!
Thanks again for all these wonderful comments. The more the better!