Can I let a bedroom or my whole flat via AirBnB or similar?

No. There are three clauses in the lease which prevent this. 

The lease specifically prevents you from subletting part of the flat.  By signing it, you agree ‘not at any time to sublet … part with or share possession of part only of the premises.’

The lease also prevents your using ‘the property or any part of it for business  purposes.’ Renting the whole or part of your flat on AirBnB will be defined as running a business.

Finally, you agreed, by signing the lease, not to do ‘any act of thing which my render void or voidable any policy of insurance maintained in respect of the building or may cause an increased premium to be payable’. Our insurers have advised us that, should they discover AirBnB or similar advertisements for properties at Hooper Square, they would re-classify Hooper Square as a hotel, resulting in a probable doubling of our insurance premium. The cost of insurance for 2019 being £88,431, the cost today could then rise to £176,000. By signing the lease, you agreed to ‘repay to the landlord all sums paid by way of increased premium … and all expenses incurred in or about the renewal of any such policy’. The company would immediately seek to recover the increase in cost and any resulting expenses from the leaseholder of the AirBnB property.