Giving a higher gas/electric meter reading than actual reading

This is going to be interesting. Another angle would be someone expecting to be out of work next month and prudently prepaying for a load of energy so they will not go into arrears..... or perhaps the user submitted an estimated reading but loaded it so the supplier would be overpaid rather than underpaid.

They're not. And you know it.

It would depend on how frequently the meter was being read. Mine is read almost every billing cycle, but I know in some cases for some reason it is much less frequent and there are long gaps (well over 12 months) between readings.

You would imagine any credit (if in fact a credit situation arose) would be raised based on the prevailing rate at the time, so the feasibility of it would boil down to how often the meter is read and the price change during this time.

As to the legalities, I wouldn’t encourage it. You are effectively submitting false meter readings with a view to making a gain, and there is an element of deception. Doubtful that anyone would be prosecuted, but I think I’d steer clear.

Everybody? Not at all. Me thinks you exaggerate a tad.

If you are going to submit an "honest" meter reading before the change in price take a digital photograph of the meter reading. Make sure that it also shows the meter number and the date/time stamp on the photo. This puts your reading above suspicion.

Indeed, and most electric meters are now read remotely anyway.