Worth a look: https://www.ordnancesurvey.co.uk/osmaps/

You can get an OS map layer on bing maps. There's also aerial photographs from different angles which can be useful for scouting trails.

I've had a subscription for the past year. I won't be renewing it until it improves a lot. It's really flaky for plotting, importing/exporting and saving routes and printing out map sections is a big faff. It's a real shame because if it worked like it should it would be a great tool.