It's 'VAT & EC Distance Selling Regulations' or something similar.
If a company is selling B2C (i.e. retail, with VAT-inclusive prices) and sells enough (I think it's a threshold of about £67,000 or something) into another EC-member country, they need to register for VAT in that other EC-member country and charge VAT at the rates applicable in that country, rather than their home-country VAT rates.
If however you sell below that threshold to that other EC-country, you charge your own local VAT rates
More on HMRC's website
The rules are intended to combat distortion of trade and unfair competition by transferring the place of supply to the Member State in which the customer receives the goods. Without this, cross-border supplies to private individuals would be subject to VAT in the Member State of dispatch as a domestic supply. With the variations in VAT rates between Member States this could encourage businesses selling mainly to private individuals (for example mail order companies) to relocate their businesses to Member States with the lowest rates of domestic VAT.
http://www.hmrc.gov.uk/manuals/vatposgmanual/VATPOSG3510.htm
http://www.hmrc.gov.uk/manuals/vatposgmanual/VATPOSG3530.htm
http://www.hmrc.gov.uk/vat/reg-how-to.htm#6
- bung 'Distance Selling' into the search box for more, but I warn you it's not exactly gripping...
For you, if you've paid the normal UK price which the website showed and yet the invoice shows 20% (if it shows 20% but is 19.6% it might not strictly be legal...), then you've not lost-out, you've paid the same amount, or even got a better deal than you would have done from a French shop, as Uncle Mort says above.
Wiggle have lost-out however, in that they are liable to HMRC (or maybe French equivalent, I forget whether UK Treasury benefits or the country of delivery) for the 19.6%, so they've made less on the item (only taken 80.4% rather than 85%).
It'll all come out in their Intrastat/EC Sales List/VAT Returns.
What they
should do I guess is ask where you want delivery to (because it's delivery address, not nationality of customer, or your invoice/home address if different to delivery address, etc that matters) and then come-up with a different set of VAT-inc prices depending where you want delivery, e.g £10 ex-VAT => £11.50 in UK or £11.96 in France.
Presumably the other retailers charging UK VAT were either below the threshold, or just plain don't know about it, or maybe their computer systems aren't sophisticated enough to cope with it
(guess what I do for a living...
)