Hi,
Don't want to be a downer, but you might struggle to rent at all.
You almost certainly couldn't rent a car in the UK - purely down to your age. Most companies require you to be at least 21, sometimes 25.
You need to check out the hire companies before you go - look at their websites and check the small print. If you can't find confirmation, then email them and be honest.
Failing this you'll have to buy, but this is not usually a viable option for anything less than 3 months (it's usually cheaper to rent for up to this length of time).
If I were you I'd look at coaches (Kiwi Experience) and trains. Or, best of all maybe - hitching!
