The idea is this: money is fungible. Put simply, that means one £50 note is the same as any other. They all have the same value and they will all buy the same amount of stuff.
This is the subject most voters (and admittedly most MP's who submitted minor receipts) were/are to stupid to understand.
The MPs who made out, but have caught no crap did this:
1. They designated one home as a second home and stuck with it.
2. They carried the largest possible mortgage on that home.
3. The walked away with with "only" mortgage payments.
4. They got into no trouble with the Daily Telegraph