I'm less confident than RoaldoM about this website. It's a criminal offence, under UK law, to trade on the internet without providing a postal address and I can't find one anywhere on that site.
The domain name is registered to
Alex Russell
18 Mansfield Road
Daybrook Square
Nottingham
NG5 6AA
but there's no phone number listed on BT.com.
The website doesn't state that the firm is a limited company but it might be worth checking the Companies House website to see if any details are listed. (I can't do it at the moment as the service is only available 0700-2400, Mon-Sat):
http://wck2.companieshouse.gov.uk/
The HMRC Advice Line can confirm whether the VAT number is valid. (I'm not sure whether they'll give you an address for the company but it's worth a try):
0845 010 9000
(0800-2000, Mon-Fri)
I recommend having a word with your local Trading Standards department. You can locate them by entering your postcode here:
http://www.tradingstandards.gov.uk/
Chris