If your intention is to stay on ‘Fibre1’ , just on FTTP rather than FTTC, ( from your point of view , the speeds of Fibre 1 sufficient, but due to distance from the cab, the FTTC offering insufficient ) , then it doesn’t surprise that this ‘upgrade’ is proving difficult ( or impossible) to arrange....there is no ( from an account position ) difference, you are on F1 and want to regrade to F1 , system says you already have F1.
The ‘obvious’ thing to do would be to upgrade to a speed that isn’t achievable with FTTC , and then downgrade to F1, this obviously carry's a cost and provides something that isn’t necessary.
It’s possible at some point, much like ‘free’ offers to move from ADSL to VDSL/FTTC , BT may at some point offer a free move from FTTC to FTTP , if retiring the VDSL estate becomes a thing, but that’s not likely to be anytime soon.
So for clarity, all the speeds BT sell can be had on FTTP, if there was no service at the property , it’s likely that a new applicant asking for Fibre 1 would also default to the FTTC offering,a customer in a new build with FTTP only could chose either , but obviously it’s the ‘poorer’ choice they are denied, you may think who would want that, but there is post on here, a person angry that they don’t have the choice of FTTC because they are in a full fibre address.
It’s possible , in your case , asking for a change of the method of delivery to full fibre from hybrid copper/fibre maybe hasn’t been built into the process, if it has apparently, you haven’t encountered anyone familiar with that process ( if it even exists ), but both you and I don’t know if it’s possible or not, If you are told by a BT representative that it isn’t , even if you think that’s ludicrous, I cannot see any other way , apart from upgrading to a higher speed band and taking that for the minimum term before downgrading to F1
