what is the process of a warrant being served my meter is outside in a locked cupboard so why right of entry? who comes to the door and how long does it take app?

hello re the above I got a letter on Friday saying british gas had got a warrant and would be fitting a prepayment meter on weds28th? they say the warrant was either served on me or sent by reorded delivery on a date last month which was infact my birthday nothing was sent or served the focus consumer group in glasgow say it cannot be stopped now how can they get away with this?