In reply to Beano:
Probably depends on your bike what you'll choose to do but I've done bikepacking overnighters (not in the UK, so not so much help specifically for your question!) which were mainly based around road riding and then others based around single track and off-road. For the former I use my CX bike and for the latter my MTB, so don't think you can only aim to bikepack on pure off-road routes that are 'bike-legal', it can also just be a cheaper and lightweight way of doing more traditional cycle touring.
Have fun, take some pictures and blog it if you find some successful routes - always great to get inspiration from others!