If you did not vote, they will send you a letter in time to come to inform you that you have been struck off the election register and in order to be reinstated and avoid a penalty, you have to get onto the website which they will tell you to apply to be reinstated along with reasons why you did not vote. No worries if your reason is genuine meaning you already have done the booking before the announced the polling day. I did that once before and its very easy to go online and get yourself reinstated.