Bear in mind that the dollar is so weak right now, that $540 (or £300) isn't that far from the mark. UK rentals usually include all the insurances. There should be a decent selection of rental comapnies in Cambridge, but also if your parents just want to go to Scotland, perhaps they should look at getting flights on Easyjet from Stansted to Edinburgh or Glasgow where they could 1) pick up an easycar, and 2) save probably 1000km of driving.

I've never used easycar, but have heard some mixed reports about them, especially with regard to over-zealous surcharging of the cars for things such as scratched hub caps. The cars also have a big orange sticker on the side advertising the fact they're rental cars, so they may feel happier using one of the mainstream companies.