You are not forced to vote in any UK national or local election, referendum, etc.
No, if you are in UK Jail at the time of any UK election you can not vote.
British and Irish citizens resident in the UK and over 18 years of age can vote in the UK. Prisoners cannot vote.
beheading
I didn't vote for Brexit but 52% of the UK population did.
jail
They vote for whoever they think will give them the best deal.
No. If you do not vote, your vote is wasted. No party can lay claim to it.
Yes, Northern Ireland is a vital part of the UK and as such its citizens have as much right to vote as anyone else in the UK.
Everyone who is 18 or over, is listed on the electoral role and is not in legal custody has the right to vote in elections - either local national or European.
No. It is not illegal not to vote in UK. There wouldn't be any chance of charging the 20 million who don't bother to vote. But it is illegal not to vote in Australia and several other countries. If you don't vote and moan, your input might have helped a different outcome, maybe in your favour.
Yes.
50 years - life in prison.