I'm not a tax expert but this HMRC guidance note seems to cover it
CG64555 - Private residence relief: two or more residences: job related accommodation
The bit you want is half way down:
Job related accommodation will be occupied either under a service occupancy or a tenancy. Under a service occupancy the property is occupied under licence and as such it is not a residence within the meaning of section 222. Therefore if the individual has only one other residence, there is no need to make a nomination in favour of the property which is intended as the main residence in due course. Job related accommodation occupied under a tenancy is a residence within the meaning of section 222. Therefore the individual may nominate which of his or her residences is to be treated as the main residence, see CG64485. In the absence of a nomination, the main residence must be established by reference to the facts, see CG64545.
It may be that the individual never actually occupies the dwelling house that they intended to occupy due to a change in circumstances or some other reason. Nonetheless, providing it was always the individual’s intention to occupy that dwelling house, relief will be available.
The key is that you have to have intended to live in that house and it's only the Service that got in the way of that. If that's true then no CGT.
Good luck!