a friend said that they were just going to buy a gps system to take with them, as it is so expensive to rent one for the rental car. but if you buy one here, it wouldn't have the map for the foreign country, right? so would you have to go looking for that once you got there? (which would seem like something you would not want to have to spend time on? also, we are wondering whether pay phones will be easy to find - for example if we need to notify a b & b that we will be late arriving, due to getting lost, or traffic, or miscalculating driving distances and times? (one day we will be driving from the highlands to the lake district; another day we will be driving from conwy on the wales coast, to the wye valley)
We rented one with the car in Spain at Xmas and was 12 euros per day which we thought was very reasonable - and it was fantastic to have and well worth it. We have since bought one as we have recently moved to Europe. I am no expert on this but I think depending on which one you buy and what you pay you can get - or download - maps from anywhere. If it was just for a holiday and I didn't need one at home, I would rent one.
If you buy one - make sure you get one thats easy to use - maybe even taker a quick free class (like REI.Com offers at its shops if you live near one). Also - check out CNET.com for opinions on various brands. You may have to download euro maps before - depending on which system. And they can be 100-500$ so depending on options - it may be the same or cheaper to rent in your car. But check out the big names like Magellan (not sure if TomTom works in europe!)
You can buy one with the Europe maps loaded. (I just did a Garmin Nuvi 370)
If you buy one without the maps loaded, you can buy the Europe maps you need in advance and load it before you go. (I don't think you can just walk into places and buy them)
Payphones do exist, but BT has been - at least trying to - remove many of them because so many people have mobiles, so you can't always depend on finding one. You're more likely to see them in places like the Highlands where mobile reception is unreliable in places. But remember that especially in the Highlands, there are few towns and a lot of empty space in between so it may be a while between phones.
As to GPS - it can be a help, but always check your route on an actual map, use common sense and if you have web access, check sites like BBC for news on road works/closures. GPSs have been known to send people on very round about or down-right dangerous routes and they don't 'know' if a road is closed. Up North, roads are likely to be narrow, twisty and you may have cow or sheep crossings, so you shouldn't let the GPS distract you and plan a route via a map first to make sure you know what makes sense.
I'll vouch for Kate's comment about a GPS wanting to send you down an unsafe road. We had one in the Ardennes Forest area of Belgium. Once it wanted to send us down a logging road. Another time, it wanted to send us down what looked like a walkway between houses, not a road. And I mean down. I would have been afraid to take a bicycle down such a steep path. But, bless its little computer brain. When we bypassed the path it wanted us to take, it computed a new route within a few seconds.