Grey imports are genuine stock. They're exactly the same products as the ones sold everywhere else.
The main issue is warranty, and very few manufacturers offer genuine international warranties. I believe Canon give this on "L" lenses, but I've never had to claim, so I'm not 100% sure about this. This means that you're facing additional shipping costs if you imported it, to return the item to the seller, if it needs warranty work or you're asking for a replacement/refund. Forget about the Sale of Goods Act. This only applies in the UK, and most of these transactions will be governed by foreign jurisdictions.
There may be another snag. I lived in South Africa for a long time, and Canon were quite happy to repair gear bought anywhere else, at their normal rates. This applied to grey imports too, because they wouldn't honour the warranty. Nikon were much stricter. They wouldn't work on grey imports at all, even if you were prepared to pay. I don't know the position in the UK. Having said that, SA prices make the UK look like a bargain basement and I knew lots of people who bought on the grey market, or imported privately, and thought it was worth taking the chance on big ticket items. B & H would just swap it out if there was a problem, but you still had to pay for the return, and they had an agreement with Nikon that they wouldn't sell camera bodies to SA customers for a while.
I'm sure retailers in this country could offer grey imports - it's perfectly legal - but I doubt if they want the hassle, because they won't have any recourse against the manufacturer/supplier. B & H are a huge company and I expect they can afford to carry any losses. US consumers also have high expectations, including the right to use something and return it for a full refund just because they've changed their mind (within limits, not all retailers do this, and some charge a restocking fee of up to 15%), so there's a marketing angle too.