Wow, thanks for all the replies! My options seem to be:
1. Send regular texts to someone, saying where I am. Needs a mobile signal though.
2. Pay for something like Garmin InReach, which relies on signals via satellite rather than mobile phone masts
3. Use a Spot device. It requires a yearly subscription but as it works on satellites and has a panic button
4. Get a dog tracker eg Tractive Dog GPS Tracker. But needs a mobile signal
5. Go for a Personal Locator Beacon which I can activate if I'm in trouble.
6. Use Viewranger buddy beacon and have battery pack backups.
My main concern is getting into difficulties and not having a phone signal to get help. I’ve been in places where even texts don’t send, so relying on my mobile makes me rather uncomfortable, as does battery drain. I can see the merits of sending regular texts though as it’s the simplest solution. Good to hear that I can expect to encounter people on the SWCP, even in poor weather/the winter, because some areas are many miles from the nearest road.
I don't know who you are, but I am getting the feeling you are doing 'Something Significant'. So why not tap into that and get sponsored by a company that sells these devices?
I suppose I am doing something pretty big, I’m walking the coast of Britain to raise awareness and funds for the RNLI and Lowland Rescue. I gave up my home and started walking from Tower RNLI station, Westminster on 3rd October and have walked continuously since, covering the coast of Kent, Sussex, Hampshire, the Isle of Wight, and Dorset up to Poole. Getting sponsorship would be ideal so I’ll try your suggestion.
I have the answer Beth! Get 2 tin cans and some string...
Nailed it!