Your citizenship is not relevant; where you live is. You need to leave the EU at the latest three months after you bought the goods. I think you may already be too late unless you are still in the EU and are planning to leave it shortly.
To quote from the HM Revenue & Customs website:
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
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.
There does not appear to be any provision for posting the form.