Yes it's illegal to kill them. Even if you kill them in the house, they will have comb and brood that needs to be removed. Rotting honey and brood will make a lovely new feature wall for the house owner. If the comb is not removed it will rot and or be a source of poison for feral or an domestic hives.
The wild life and country side act 1981 - this is the actual killing of them
The control of pesticides regulations 1986 - about how they would be killed as the pesticides are not likely to be sold or licensed for that purpose.
I would explain that they could be removed, and say you'll get them out, but are not responsible for the removal / replacement of bricks or what ever to gain access, nor are you liable for anything that happens as a result of that process. I would get that in writting. it is not illegal to kill bees