Hi
I doubt HS is a genuine UK business because a genuine UK registered business publish their address and working phone number.
Payments goes to Hong Kong.
Royal Mail 1st class recorded delivery doesn't take several days to arrive but it does if it's sent from Hong Kong, I know by personal experience.
They don't supply the tracking number for the same reason as above.
The five star reviews on this site and another are 'placed' and therefore faked. Two other businesses who's selling teeth whitening have reported Happy smile as malicious, coming from Michael Greenwood himself. You can check it yourself by clicking of a five reviewer's profile and look for their other reviews.