Hi,
Just to help, it would make no difference if there had already been fibre to a building as unlike broadband and phone lines, the fibre install is proprietary and belongs to the person that installed it, so let just say the prior occupier had a fibre leased line from say C&W and then you went to AN Other provider they would not be able to use the existing fibre as it does not below to them so they would have to run a new feed.
What will make a difference for the install is the distance from the building to the nearest POP (or some carriers we use, will quote to their nearest pit)
Install cost will be from termination point to POP (maybe exchange but not always) and some carriers we use if they have say a fibre feed across the road, will then charge from the termination point to the nearest road pit where they can pick up a dark fibre feed they have.
There really are so many variables for fibre, some will do a fixed cost install and others will charge for the fibre length.
At the end of the day almost all providers will give you a free quotation, and once you have a few then you can see if it would be worth it or not (also don't forget the option of sharing a leased line is a possibility, we do this quite a bit)