Well I have been told in another post that they decide where it goes, which kind of sucks, having a fibre cable trailing through a house is a bad idea.
I say explain to your ISP before you order FTTP where "you" would prefer it to go, and also explain this to the engineer(s) when they arrive and see what they say, you might be lucky.
I'm not sure explaining anything to your ISP about what you would prefer would do much good, but it's definitely advisable to be present for the first stage appointment especially if you want a 'non standard' installation, which goes without saying if the CSP is to be fitted internally.
The easier you make it for the installer to achieve your request, the more likely it will be complied with.
In my case (supported by a comment by TBB member Zarjaz) I fitted a continuous flexible metallic conduit through the loft, down via a chased-out wall, then under floorboards etc. to a downstairs (indoor) utility cupboard discussed in this post and following posts
(viewed threaded, i.e. not flat view) where I had the CSP fitted.
The 'internal' fibre was then routed through a flexible plastic conduit to the room above where I had my ONT fitted, before being spliced at the CSP. Excluding inside the utility cupboard, total visible fibre inside the house is less than 150mm where it emerges from the wall and plugs in to the ONT, and when the OR installer had finished he commented that he wished all his jobs were as easy as mine!
Bear in mind that you must stick to the guidelines in the OR Developers Guide regarding bend radii etc.