Open the tacho and set the time on the clock to the correct UK time. Use that time until you have BST or leave the UK. If you leave the UK keep the time until you change the disc then align the time with the new country's time.
If you find the disc is already in the pot and then you see the clock is wrong, continue on the wrong time but ensure your totals are not exceeded. At the end of the day certify on the back of the disc in black ink that the clock was wrong and was fast slow by x minutes. Sign date and time it and get it countersigned by a supervisor.