1. There is no suggestion that the cars can be viewed or test-driven, or independently inspected, before purchase. No one in the UK sells a car without making it available for inspection, and no-one in the UK should ever buy a used car without inspecting it (and unless it is an auction, without test-driving it).
2. The address given looks on Google Street View to be a house on a residential street. If it was a legitimate company the address would be at minimum an office, if not a warehouse where the cars are stored.
3. Nowhere on the site is there a Company Registration Number, showing that the company is registered with Companies House. There should be a Registration Number and a Registered Address (which isn't always the same as the normal business address).
4. If you actually look up the company on the Companies House website, you will find it does exist, but its total assets are about the value of 1 used car, not the amount of cars they claim to have in stock. Given the company is registered, the only reason for not putting the Registration Number on the website would be to discourage people from checking the Registration and accounts.
5. According to some users on the MoneySavingExpert forums, the same vehicles existed on another genuine car dealer's website.
I don't think it is a genuine sales site.