rockhopper wrote:I also carry a cheap vodafone phone as occasionally it gets a signal when O2 doesn't.
Think that's the best answer. Voda and O2 are the two I'd go for, costs £10 (or less if you have an old phone knocking about) to get a cheap phone that'll last days on standby.
If using it for mapping then it's worth getting the paid content for Viewranger that's stored on the phone, all UK national parks can be had in 1:50000 for a tenner if you shop about. It's less faff than saving OpenCycleMaps onto the phone for offline use.