Boiler location

Yes. A gas appliance may be fitted in a bathroom provided that (a) it is room-sealed, and (b) the electrical zone requirements are complied with

In England and Wales, Prat Pee will also apply, but many (not all) CORGIs should be able to self-certify.

Owain

Yes, subject to the cupboard meeting the manufacturer's specification for minimum size and ventilation requirements.

You can fit one not in a cupboard providing it's a minimum distance from a bath/shower (60cm IIRC) and on an RCD (IIRC). (Don't have it share an RCD with anything else though.)

Yes, mine has been in the past week by a company and is perfectly OK. The flu goes out through the wall. You can fit them in a variety of places, cupboards being favoured to keep them hidden. They can even go inside the loft if you want.