FlyingForFun...you're from the UK, I'm not and I don't know whether the pilots at outfits like BA or VS are rostered or whether they can choose their lines of flying according to seniority.
But in the USA seniority rules. Pilots are not "rostered," (arbitrarily assigned flights), they bid for available monthly lines of flying (trips) as published by the company. Union contracts usually include a "sheduling committee" which, needless to say, usually is made up of senior pilots who endeavor to build favorable trip combinations.
What that implies is that a senior pilot probably isn't going to bid a night flight from ANC to ORD in the dead of winter, so as to give an opportunity for a junior pilot to bid a choice day flight from LAX to sunny HNL. It implies that a very junior pilot may not see HNL for many years; it implies that a very junior pilot could be stuck in Africa, living out of his suitcase most days of every month for many years.
Well, that's just the way the cookie crumbles. Why should a junior pilot who may be a favorite of management be given choices over pilots senior to him?
Seniority does not mean that a copilot "next-in-line" will automatically be upgraded. We have copilots who are senior to captains, and we also have permanent copilots.