Sua cidadania não é relevante; onde você mora é. Você precisa sair da UE o mais tardar três meses depois de ter comprado a mercadoria. Acho que você já pode estar atrasado, a menos que ainda esteja na UE e planeje deixá-lo em breve.
Para citar o HM Revenue & Site aduaneiro :
If you're travelling outside the EU, you must show your goods, receipts and your refund form to UK customs staff at the airport you're leaving from. Make sure you arrive at the airport early so that you have plenty of time to deal with the customs staff before your departure.
If you're travelling to another country within the EU before you finally leave the EU, then you must show your goods and refund form to customs officials in that country when you leave it.
...
If there aren't any customs officials at the port or airport you're leaving from, there will be a telephone you can use to ring an official or a clearly marked customs post box in which you can leave your refund form. Customs officials will collect it from there and if they are satisfied that all requirements have been met, they will contact the retailer to arrange your VAT refund.
Não parece haver nenhuma provisão para postar o formulário.