Regarding automatic vs. manual transmissions:

I think you can work with travel agents here in the States who will link you up with one of the big car rental companies, and reserve a car for you that has an automatic transmission.

But you can get a much better deal if you wait till you get to Britain, get settled, recover from jet lag, then look in the Yellow Pages under "car hire" (maybe someone in Britain can confirm this). Many auto dealers and repair shops have one or two rental cars available. Trouble is, you may have to take what you can get. It will probably be a small (by American standards) efficient car with manual everything. But since you're traveling in the off-season, you shouldn't have any trouble finding SOMETHING.

If you already drive a manual transmission, it won't be that bad. At least fumbling with the gearshift is not likely to cause an accident.

But I urge you to consider public transportation instead.