Transfers within the UK (local currency to UK account) currently are forced to FPS, for which Bunq charges a fee. Yet, the UK is in SEPA, so it makes sense to support that and do a transfer to an IBAN like with any other instead of doing it differently.
As a side note, while SEPA isn't yet possible: After entering a UK IBAN as the destination, one gets asked for the sort code and account number. Those are part of the IBAN, so there should be no need for those to be entered again manually by the user.
https://en.wikipedia.org/wiki/Single_Euro_Payments_Area