Last time, anyone returning after taking the redundancy package had to re-join and started again on the seniority list.
True enough, loosing 3 years seniority pretty much finished off a few careers although it was not apparent at the time. The pill was sweetened somewhat by taking previous service into account, so someone with 2 years prior service came back into year 3 pay wise (Had to thank BALPA for that) although at the bottom of the seniority list.
No idea what the situation would be this time around.
If you do join, go on the Airbus, especially if you plan to commute. The Boeing only works if you live 5 minutes from LGW. And you will still be knackered. Long haul flying on a short haul roster was they way it felt to me. I don't believe it has changed all that much.