Pasta:
Some older meters only function 'locally'. i.e. they monitor how much gas or electricity has passed through them and know how much credit has been added to them but none of that information is sent directly to the energy company.
Newer meters transmit signals to and from the energy company, with that company knowing how much credit is on the meter and with that company being able to add further credit to the meter. If your meter 'magically' gets the Warm Home Discount amount added to it, it must be one of the newer types and I'd expect something similar to happen with the £200 rebate.