If you've been registered to vote on the electoral register in the UK within the last 15 years, and you're now living abroad as a full time resident, you're entitled to register as an 'overseas elector'.
If you've lived abroad for more than 15 years, you cannot vote in UK elections.
If you move back to the UK as a full-time resident, you can register to vote using your permanent address and be granted full voting rights.
Also see
Electoral Services
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