Surely taking the rad out (fairly easy) would be enough, I don't think there are regs specifying what a non-habitable room should provide, just what a habitable room should comply with.
If it's not got a proper access and no radiator, it can't be classed as habitable can it?
The only other thing I can think of is planning, but if it's not habitable then you haven't changed its use.
A quick google suggests that in addition to PP enforcement not being carried out after 10 years (change of use), Building regs don't normally bother after a year (and that's for habitable spaces).
Looks like you live in NI so maybe the rules are slightly different?
I'd be tempted to tell them it's not used as a habitable space and wait to see if they try to enforce, maybe take the rad out but certainly not tear the boarding out.
I did tell him it wasn't to be used for habitable space but he said because it had the boarding in it wasn't sufficient to just take the rad out, it all had to come down as I could easily change my mind and use it to sleep in. He was even going to get me to lift the carpet!!- until he had a change of heart.