I need an external Samsung T7 to boot from and there are some great deals on Amazon (UK). However, given the importance of an SSD is it a bit risky?
Specifically I mean are there any counterfeits of this sort of thing or otherwise dodgy items on Amazon for this sort of product?
There are counterfeits of most everything everywhere on Internet vendors.
That being said, I’d trust Amazon and when purchasing I take note of the supplier (which Amazon displays) to make my own judgement. And if you find the product is counterfeit and/or doesn’t work, Amazon excellent at doing the right thing. And buying with some credit cards in UK provides additional failure “insurance”.
But it’s impossible to really answer your question without dropping into generalities and non-specifics like I probably have done. 
I’ve bought a T5 and Sandisk Extreme from Amazon UK with no issues at all.
just make sure it’s sold and fulfilled by amazon
Same as aardy above - I’ve built up a number of SSDs all from Amazon UK - mostly Crucial, but some SanDisk - all have been legitimate and worked fine so far (touch wood) - some I’ve had a number of years.
As the other posters have said, I always check it’s being sold by Amazon (or the manufacturer perhaps), rather than some random seller. Amazon is good at taking things back if they turn out to be faulty, or not as advertised.
I will add I’ve had SSDs in both normal “retail” boxes and Amazon’s “Frustration Free Packaging” which is basically a more eco-friendly brown cardboard box usually, rather than a pretty printed box - and they’ve been exactly the same, aside from the box, as the retail packaged product - coming with the SSD, mounting bracket, paper-leaflet, and I’ve not had any issues with any of the SSDs regardless of packaging.
Hope this helps 
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Yes, there are but if you check sold and fulfilled y Amazon or the manufacturer and it’s odd you can get it replaced easily. I’ve bought several external SSDs and the internal one for my iMac off Amazon.
One caution, I had to buy a dozen thumb drives in the shape of a black sheep. Many failed, Amazon replaced them but more interesting was that nearly ALL of them were infected with various bits of malware. All went first into an airgapped Linux computer and got fully reformated before being placed into any working machine.
So to be extra cautious, I did test my external SSDs on a machine I had a good current bootable BU of with it disconnected from the internet first. Nothing found on them.
Just as a note, I’ve bought some drives from B&H Photo & Video and found them to be good. Doesn’t seem quite as “potentially sketchy” as Amazon. Prices can be better or worse depending on the day.
I’ve gotten stuff from Amazon too. Generally though, the better the deal the more I scrutinize who it’s sold by.
There are dodgy drives on Amazon. My mom got two. They were from an independent seller and were not fulfilled by Amazon. I’ve bought at least half a dozen drives from Amazon with no problem (including a Samsung T5). I just make sure the seller is the drive manufacturer and that it is fulfilled by Amazon. So as @OogieM recommended, just pay attention to who is the seller and who it is fulfilled by.
I’ve purchased several Samsung SSDs from Amazon without issue. Mostly they have been either 850, 860 or 870 EVO SATA drives that I have installed in external enclosures, although recently I purchased a couple of 970 EVO Plus NVME SSDs. All of them have been official Samsung new devices and worked flawlessly.
However as an alternative, I have found prices at B&H Photo to be very competitive with Amazon. Shipping is a little bit slower, but not terrible.
I’m in the US however. I have purchased a couple of things from Amazon UK, but not any SSDs. I would like to think their practices are similar, but I don’t have any evidence to support that.
Sadly, they aren’t in the UK
Is there a way to test if drives (SSD, HDD, USBP) are counterfeit or fake?
I’m more wary of USB drives and I think having an airgapped machine that formats them makes sense.
I’ve bought about a dozen Samsung T5 and T7 drives on Amazon and all but one has been top notch – and the bad one was messed up by Catalina Disk Utility, so it probably wasn’t the hardware’s fault.
I bought all my Synology drives from B&H – but I won’t mention how long ago since they’ll hear me and decide to fail – but it’s been a good long while.
Do you have any huge online electronics stores over there that don’t allow third-party sellers and have decent prices? That’s basically what B&H is over here.
The issue with Amazon isn’t so much Amazon as it is all the third parties on their platform - and some rumored questionable practices where they put Amazon-fulfilled products from, say, Seagate Direct and Jimmy’s Discount Drive Hut into a big, comingled bin because Jimmy’s pinky-swears that their drives are legit. And then when somebody orders a legit Seagate drive, they put a Jimmy’s drive in the package because that’s at the front of the bin.
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Any retailer that doesn’t do that would be good.