I imagine that they assumed that head-tenant's consent to the assignment of the sub-lease would be automatic and so they just went ahead .... then realised that NatWest don't actually care whether the unit is occupied or not as the rent is so low that property staff in London can't really be bothered to help or just don't want a start-up business going in if they can keep the certainty of wiltshire College paying the sub-lease rent.
By law, Landlord's consent is not to be unreasonably withheld but the cost of going to court and the delays would not be worth it if there are easy alternative units to take instead ...
They are dentists ... not hard-nosed property experts ... and judging by the surnames (mainly asian) English may not be their first language ....