Others have recommended buying the Ordinance Survey maps available at petrol stations and convenience stores. I'm sure they are very helpful, especially if you need to find places that are not major landmarks or located on back roads. We did fine with Google Maps researched at home. I printed out maps for specific locations like the hotels we'd reserved. Then, of course, we could always call the hotel when we were within half a mile or so and get pointers on what signs & landmarks to look for.
As you probably know, you can get GPS in a rental car -- but if you're not familiar with how they work (as we weren't), it may not be much help. For example, one of the places we needed to go had a postal code of just two characters. The GPS would not accept it, and when I tried adding a dummy character like zero or 1 it wouldn't take that either. We would have been happy with a general map of the overall area, but there didn't seem to be away to request that. Another time, we wanted turn-by-turn directions to get through and out of a town, but the GPS kept sending us around in circles.
You may ask why we rented a car with GPS if we didn't feel comfortable using it. The answer is that the particular car we got came with GPS; we didn't ask for it. (We're probably the only American family that doesn't use GPS in our own cars, but that is our choice!)