To vote in the UK you have to be 18 or over, and you need a polling card or need to have arranged a postal vote.
Non-British EU nationals living in the UK can vote in all local elections so this includes borough, mayoral and Assembly.
If they fill out a UC1 form (obtained from the borough you live in) they can also vote in the European elections.
In 80's followed by the ethnic violence in Sri Lanka, Tamil Sri Lankans started migrating to UK. Tamil Sri Lankans might make around 0.2 % of UK population. The people from other south Asian countries would make around another 0.2% of UK population.
But non-British EU nationals can not vote in a UK parliamentary election. However, people from Commonwealth countries can.
People from Ireland, Cyprus and Malta can vote in UK parliamentary elections as well as the local elections as these countries are considered to be Commonwealth countries.
To vote in the London local elections you have to live in London.