They appear to based in New Zealand:
http://www.nominet.org.uk/whois/lookup?query=Vetmeds4Pets.co.uk
(If they were based in the UK they'd be trading illegally because their website doesn't provide a postal address, which is always worrying).
If you buy from them you risk having problems if your goods aren't delivered or possibly (if your order is for prescription-only items) having them impounded by UK customs. Further (depending upon exactly what you order and its value) you might get hit for Import Duty and/or VAT. If either of those apply you'll also have to pay an £8 customs examination fee.
Most of the items they offer seem to be the type of things that pet shops aren't allowed to sell but which are available (without prescription) from pharmacies. For example, Asda pharmacies offer some of the best prices on flea treatment products.