As previously announced, the British Authorities have now confirmed they hold online platform jointly and severally liable for the UK VAT owed by resellers if they don’t meet their VAT requirements
(see HMRC notice here: https://www.gov.uk/guidance/vat-overseas-businesses-using-an-online-marketplace-to-sell-goods-in-the-uk#JSL).
Some platforms such as Ebay have already announced their full cooperation with the local authorities in order to have sellers register for VAT purpose and pay UK VAT.
After France, UK has set up the appropriate tools to tackle sellers not complying with (UK) VAT obligations.
Source: VAT News N° 5 2017