What is the legal situation regarding double sided sandwich boards that go over someone's body and they walk around with? I presume they are totally legal as they are not a fixed object you are leaving on the street?
You have to keep moving so it is not considered a stationary billboard; however having said that if your feet are not screwed to the pavement then I suppose nobody can say you are fixed in your ways.
Every council used to keep a list of controlled streets where for a licence fee you are entitled to apply for a licence to distribute free literature and can usually pick your spot.
So you might be able to wear your sandwich board as well as give out leaflets now and then. This will enable you to stand in one place with your board fully licensed to do so!