I reported earlier that 1 out of every 4 or 5 packages of contact lenses purchased (without VAT) from the USA get flagged at customs. In this case, you will pay an extra 2% import duty, and additionally an extra 17.5% in VAT. In addition (although we've never heard of this happening), you "may be liable to financial penalties or criminal prosecution". Read the note about "unscrupulous overseas suppliers" in the HM Revenue & Customs "Shopping on the Internet" page.
DO NOT BUY from US companies unless they clearly state that they will charge you VAT and import duty and will correctly fill out the customs declaration!
They compete favorably on prices, but do not mention that what they are doing is basically leading you into illegal activity, which only YOU are responsible for. In the end, you may get lucky... or maybe not.