I realize this site is UK centric, but my experience in US ATC is the FP alternate is just a technicality, like showing your 8th grade math teacher how you arrived at an answer on your homework. Once airborne, and confronted with the need to divert, the typical airline crew consults with dispatch and makes a decision based on actual fuel remaining and existing weather, not what was forecast 4-6 hours ago. As a US terminal controller, I never had a clue what the crew or company filed as an alternate. Never saw the FP, only the clearance into my airspace.
One of the things I taught my instrument students as an instructor pilot was that you were never "locked" into going to your filed alternate. Tell ATC where you want to go next, and they'll do their best to route you there.