The Representation of the People Act 1985 (as amended) provides a facility for British citizens overseas to retain their voting rights if they have been resident in the UK in the previous 15 years, so long as they are not otherwise subject to a legal incapacity to vote. The Representation of the People Act 1983 makes convicted prisoners detained in a penal institution (or unlawfully at large from one) legally incapable of voting in a UK general election. However, ‘penal institution’ is defined in the 1983 Act by reference to Prisons Acts, which do not extend to foreign jails.
Accordingly, British citizens imprisoned in jails overseas are eligible to vote in UK general elections if they are otherwise eligible to vote as an overseas elector under the relevant provisions of the Representation of the People Act 1985.