Apart entirely from the infosec issues, in a polling booth no one can see your actual vote. Whatever else is wrong with it, in a polling booth no one knows how you voted...no matter who or what is pressuring you outside. With any form of remote voting (snail mail or otherwise) there is no way to verifiy that the person is not being observed by someone. Perhaps that is not a concern for rich world citizens too busy to get engaged but there are many parts of the world (and no doubt some parts of the UK) where pressure is brought to bear on people.