Hi there
1. I can't add anything to the driving answer, other than to totally agree that it's best avoided in the centre of our major cities if you're unfamiliar with the territory (and even if you are familiar

).
If you do have a car, most of our large towns (but not the big cities because they are too big) have Park + Ride schemes (some better than others) which consist of a large, out of town car park with a bus service into the city. Your parking charge includes use of the bus.
2. Youth Hostels have a very safe reputation but, as Vernon says, are very basic - that's how they keep the cost down. However, they can be fantastic value and can be found in some stunning locations. You'd really need to do your research to check individual hostel facilities as they are all different.
If you're planning a summer trip, another inexpensive option is to rent rooms in our university student accommodations when the students have all disappeared for the summer break (usually sometime in June to late September). These can also put you in great city locations, but you'd need to check out exact facilities and locations on offer.
3.
IMHO the best value accomodation is often found in the B+B (Bed and Breakfast) houses
Just wanted to point out that UK B&Bs aren't necessarily the same as US / Canadian B&Bs. Some are very luxurious, but some can be less appealing. From what I've seen of the US / Canadian ones, they'd be like the better ones in the UK.
Good luck with your planning!