But planning was for a garden room, so the regs would have been assessed for it being a garden room. Just because it was then used as a bedroom, it does not mean that it would have passed the regs as a bedroom (if there as such a thing at the time, which is what I think you're now trying to find out).
Are you now wanting to sell the flat as a 2 bedroom? If so, I'd first be checking the freehold agreement to see if the freeholder has a basis to block that. If not, you effectively have two choices: 1) Advertise it as a 2 bed and see what happens with the risk that if someone complains and you're forced to re-advertise as a 1 bedroom, you'll likely have to reduce the price or 2) Go to a lawyer and pay them to make an assessment on whether it can be sold as a 2 bedroom, and if not, at least you're in a position of knowledge of where to price it in the market.
I guess there's a 3) too. Post all the details here and risk taking free advice from randomers on the internet, even if they are some of the smartest ransomers out there 😀