Did the solicitors acting for you (HSBC panel) give any advice on the lease at all particularly with regard to the use or any other points in the lease? How would they have known some stage in the future, you might want to sell up and change the use completely? They give you a report on the lease?
What is onerous about the lease? Just the fact that it is A1 use only? Does it actually say in the lease that this is A1 use only or does it say “or any other use which the landlord may approve”? Does the brochure SAY A1-A4 use?
Does it say anywhere that consent is not to be unreasonably withheld or delayed?
If you can attach the lease, I am happy to tell you whether these clauses are in because I will obviously know where to look.