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Yeah, there's that risk. I'd suggest there'd be a bias in their favour when applying, not a guaranteed job waiting for them. And if you clearly fucked up as an MP, then you can forget about it, the local constituents will know about your poor performance better than anyone else.
The job would have to be in the constituency you represented. For two reasons; this means you can't use the preferential access to just stick around in London because you can't be arsed to move back. Also, you were the person the constituents voted into parliament, so in most cases you must at least be reasonably competent compared to the local competition for that job.
And there's class of course. Someone who didn't grow up with connections will have a much harder time deciding to take a break from their decently paid engineering career (or whatever) for an exhausting gig halfway across the country that could spit them out again in four years time.
Meanwhile mediocre upper middle class MPs who are likely to lose the next round of elections can just start putting out the feelers in their network and land a cushy PR gig.
So yeah, taking that risk should be compensated for. Maybe a perk could be preferential access to civil service jobs back in their constituency for MPs who doesn't get re-elected?