Ultimately it depends on HMRC!
A mates accountant lets him claim stuff that mine doesn't. Another friends accountant agreed for him to claim something that might be questioned with the proviso that if HMRC came a-looking, then he might have to pay it back!
If HMRC never look at you, then all the above is irrelevant. Thats why some people have claimed things back successfully while others have been advised not to.
Plus there are differances between the truly self emplyed, and those working through their own limited company (me).
I am still waiting for a statement from HMRC as to how far I can modify (convert) my panel van before incurring extra tax due to it being considered "dual purpose". I'm not holding my breath. As yet I have no wondows and no rear seats, but I am coming to like the "stealth camper" approach. I have no private use (though that may change in the future) only using it for commuting to work, and stopping overnight on occasion. I have been upfront to HMRC about this, and the intention to also use it as a mobile office (if I got seats and a table), because I don't want to suck-it-and-see.
But for others, if HMRC don't investigate your "affairs" within six(?) years, it doesn't matter does it?