Become and M.P until you get a better job

It seems to us that if someone is elected as an M.P they should (with the exception of illness or death) remain in their post until the next general election.
The costs to the local ratepayers and taxpayers of running an election runs into thousands of pounds, and is a waste of money, which could be better used on local facilities, health etc..
Just this year 2 M.P’s have resigned their posts in search of a 
better job.
Labour M.P Jamie Reed announced last month he was standing down as MP for Copeland to take up a post at the Sellafield nuclear plant, and now Labour MP Tristram Hunt is quitting as an MP to become the director of London‘s Victoria and Albert Museum, triggering a by-election.
No doubt all M.P’s state when they win that they are committed to working as an M.P for all the people in thier constituency, but they don’t mention….just until I get a better Job