Car hire manual or automatic

If you can drive a straight shift, don't hesitate to get a manual in the U.K. You'll immediately get used to changing gears left handed.
Most of the rental cars are hatchbacks, but they have a rolling vinyl cover that covers up your luggage from eyesight.

The shifting pattern is exactly the same as right-handed - it's not mirror image - so it's easy to get used to shifting with your left hand.

I have driven in England and Ireland, a couple of trips each, and have always gotten an automatic. For me driving on the left is plenty to handle even though I know how to drive a manual. The cover with a hatchback may not do as much to obscure your luggage a real trunk, but you are unlikely to get a real trunk on a rental car in Europe based upon my experience. I think the hatchback cover is enough though, as long as your luggage fits under it. Our 25" bags were took big and the bulge in our compact car was fairly obvious on our 2013 England trip, but we chanced it and got away with it. Going forward we will always travel with 22" bags especially when we plan to rent a car. On our 2016 Ireland trip the 22" bags just barely fit without showing under the hatchback cover of our compact car. The space for luggage is very small so pack accordingly.

I have driven most of my life driving a manual transmission car in the US. When we went to Ireland for 2 weeks we rented a manual transmission and I had absolutely no problems shifting with my left hand. Within a day I never thought about it again. The only problem I had, and this would apply to automatic also, is that I was always looking to the wrong side for the mirror.

LOL, Bob. I kept trying to put my left elbow up on the arm rest.

I've had the same experience as the others who've already posted. I've driven nothing but manuals in Britain. If your nervous about it, spend time driving on out of the way roads or quiet neighborhoods to get the hang of it and then head out to wherever you need to go. It's really not a problem.
There are plenty of cars with "boots" or covers over the luggage area if it's a wagon. I would definitely get something where you can cover your belongings if you need to leave them in the car. I've never had a car broken into in Europe but it does happen, as it could anywhere.

Check for price breaks for rental based on the days of use. For example 21 days, 15 days, etc. You may be able to save money by taking a "break" in the rental. Also check to see if day of the week the rental starts and ends makes a difference. This was a little while ago so we can't quote current prices. Also check fly drive packages. We did a month in UK but spent 6 days in London no car. Then rented one from Heathrow returned to Gatwick for 15 days. We rented a automatic compact which held 2 smaller wheeled bags. Many have navigation now. We didn't worry about our luggage in the countryside. Just take ugly luggage and never leave your electronics or cameras behind. If you are at major tourist sites you might be a "mark" because of your cars license plate. One strategy is to arrive at you destination early and check your bags in before site seeing.

Our itinerary in England and Wales doesn't include London and the largest cities we visit are Cardiff and York. It is going to be a great trip, we have booked mainly B&Bs that provide parking so we don't have to pay for parking.
I will insist on a navigation system, but still bring a good map.

If you are experience driving manual, then don't hesitate to go with a manual transmission in the UK. It really is not the terrifying experience that many project. You adjust very quickly to using your left hand for shifting. The pedals are the same. There are many other aspects of driving in the UK that are more challenging. If you have a navigator who can help you with the other aspects then I say go for the Manual and spend the money on some other part of your trip.