Does anyone know if Openreach will run a fibre for an FTTP install through a loft?
It all depends on the loft detail ..... if it isn't adequately boarded out and with proper lighting, then the engineer should not be entering the area under the 'confined spaces' mandate. No amount of debate will change that fact, it kind of falls to the engineers own risk-assessment as to whether it is adequate or not.
But, certainly not worth them facing severe disciplinary action should something untoward happen.
As mooted though, if you fed your own conduit with a draw-string ready to attach the fibre to, I couldn't see a problem at all personally speaking. As Ron also mentions though, make sure the bend radii aren't too severe !!