The standard install requires an external CSP to be placed at ground level.
For an overhead feed where you want the cable entering the attic that means the drop cable runs to the property, down the building to a CSP and then back up the wall to enter the attic.
Only a good/flexible engineer will be willing to do that. Openreach use subcontractors for much of their FTTP installs and they likely won't be willing to do this, instead only offering a downstairs ONT placement.
They are paid per job so are only willing to do standard installs.
Even some Openreach employed engineers won't go near your attic.
There are such things as internal CSP's that in rare installs can be put directly in the attic meaning no need to go down/up the building.
Getting this as part of standard FTTP install is next to impossible, often requiring turning away engineers until they send 1 willing to do what you ask, and only if they can source an internal CSP.
Openreach offer an advanced FTTP install that costs more and allows a longer run of fibre between CSP (ground level) and the ONT where you choose.
Few mainstream ISP's have the ability to order the advanced installs and even then they don't guarantee you can get an internal CSP in the attic.
In short, while you can hope for an internal CSP in the attic you should plan for a ground level CSP.
Getting the ONT installed in the attic should be easier but preference from Openreach would be ground level.
