This is a scam which has unfortunately been running for a while.

This is what we've since found out about prepay electricity meters:

You get a warning which informs you that you're low on credit. If you ignore that warning and your credit goes below 50p you can activate £5 of emergency credit. £5 of electricity credit will last the average household a reasonable amount of time in order to go and top up. Therefore, there should be no real reason to go door knocking for money.