Bashfulclam you friend has been briefed incorrectly. The presiding officer keeps a running total of votes issued as all the ballots are numbered. This total is checked at the end of the day and verified against the number of votes in the box. Recounts are done until the number matches. The elector number is written on the back in case anyone challenges the result. The court would have to order the scrutiny of the counted papers to be matched against the corresponding list of numbers from the polling station. It would be a huge job and I don’t know if it’s ever been done. any electoral fraud would be extremely hard as
three officials in the polling station who in many cases would have only met that morning. any voter can walk in at any time, there are independent observers and the box is sealed etc.
for the person who arrived at the polling station to find that their name had a mark on the register to say that they had already voted it’s likely that this was a mistake by the polling staff rather than someone impersonating you. It happens very rarely ie about one case across the whole borough.