I'm a non-EU citizen. I left London from Luton Airport at 23:30. I did not find the blue HM customs post, so I took the VAT refund paper with me. How I can request the refund? Can I send it by post? To which address?
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 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.
There does not appear to be any provision for posting the form.