The main criteria be registered to vote are:-
The application should be for the address at which you currently reside, that being your usual and permanent place of residence and not a temporary residence such as a holiday home or second home.
In Scotland, you can register to vote when you are 15 (and in some cases when you are 14). You can vote in local and Scottish Parliamentary election when you are 16. The voting age for UK Parliamentary (Westminster) elections is 18, however you can register to vote when you are 17.
Persons eligible to be registered are:
Commonwealth Citizens who have leave to remain in the UK or who do not require such leave
Citizens of The Republic Of Ireland
(The citizens listed above have, subject to any other statutory restriction, full voting rights at all types of elections and referendums).
Citizens of Other European Union Countries
These citizens are entitled to vote at Scottish Parliamentary and local Government Elections only, although they may be included in National Referendums depending on the relevant franchise restrictions for the event.
EU citizens are not eligible to vote in UK Parliamentary Elections.
Further information is available at https://www.yourvotematters.co.uk/can-i-vote/who-can-register-to-vote