I believe it is that case that the UK government does not insist you have a passport to travel. Practical considerations may make it difficult without one (many private companies, for example, won't let you onto their conveyance without one because they don't want to be lumbered with taking you back again when the destination country refuses to let you in for lack of passport) and the destination country might not let you in without one, but that's not the UK government insisting you be licenced to travel. It's not illegal under UK law, I understand (or it certainly didn't used to be; the maybot did seem to run an awfully authoritarian Home Office so maybe that's changed), to simply wander down to the sea, get on a yacht, and sail across to another country.

I have myself gone to France without a passport (which may have been illegal under French law; I certainly have the right to travel there freely, but I don't know what they insist people carry), gone shopping, and come back again, and I know someone coming back through border control who simply gave his name and address.