I plan on taking my DSLR and various lenses but there will be a couple of days round the beach where I don’t want expensive camera gear left unattended.
Don’t want to pay more than £100 also like to use it when I return when out on the mtb.

For on a beach or mountain biking, I think a waterproof camera is a good idea.
I recommend the Panasonic Lumix FT25. It is waterproof, and reasonable quality photos. The latest model is the FT30, so the FT25 is now a bit cheaper, probably about £100.