The vote should be available to anyone over 16 who is settled in the United Kingdom, with the exception only of those who have such a serious mental disability that they are unable in any way to understand the process they would be involved in. This is because we are all human beings who are subject to the decisions made by those who are elected.
People should not be excluded from the vote on the grounds that they are in prison or are nationals of a foreign country (in this category at the moment only Commonwealth and Irish citizen can qualify).
Serious action should also be taken to ensure that the missing millions who are entitled to vote (but who are missing from registers) are discovered and added to the registers. Those missing from registers are a significant proportion of the poor, the young, the rootless, the homeless and ethnic minorities.
A major case for giving the vote to 16 year olds is that they could initially be registered via their schools when 15 as "attainers" and it would then be easier to trace them later to ensure they then maintained their voting rights.
I can not understand anyone who otherwise sees themselves as a democrat and a believer in human rights coming up with reasons to exclude any category of people. The vote is not something that one should earn. It is for politicians to seek to earn our votes.
As things stand with criminals, we have the odd position that we keep discovering prisoners who are innocent whilst we know that most criminals are never captured. If there was a justification for excluding prisoners from the vote, (which there isn't) we would need a superior justice system to the one we have - yet it is currently being subject to Government cuts.