Here’s an example of some GPS units that may work for you. For a broader range check out this link: Handheld GPS It’s not an exhaustive list by any means but its a place to start. Just stick to your budget. You should be able to get something for under £100. I spent a bit more to get the onboard maps function. The Garmin eTrex Summit:
It comes with maps installed but you will quickly realise that you need more detail. So download and install the OpenCycle map, which is freely available from http://talkytoaster.info/ukmaps.htm More information on Open maps go to Wikipedia http://en.wikipedia.org/wiki/OpenStreetMap.
And some others that may work for you: