I don't understand why this doesn't work in a similar way to when a prime minister/party leader steps down and another is elected up from within. Say an MP goes on maternity leave for 6 months why can't the party field an interim MP from their candidate pool? So if a labour MP was elected in a constituency and they go off on legitimate leave then a representative labour MP steps in, they should theoretically promote the same values etc, and they could confer with the MP who is off on large decisions. They would then step down when the MP comes back.