Ethernet cable – 25m – Internet, patch & network cable with break-proof design for maximum UK internet speeds £6.79 @ Amazon

Nope, I would avoid, it's not copper and really is in breach of trading standards as Cat6 cables must be made with copper to be labelled Cat6. Sure you get a network connection using it, but it will introduce that much jabber on your network it's not worth it. It does state it's 581 grams, current UK copper prices would see this as around £1.50 worth of copper. So a 25m copper cable can be had cheap enough, but this is not it.
The only way to be sure is to buy it and test it yourself - the easiest test is to hold the bare wire under a flame if it doesn't react, chances are it's copper, if it catches fire and spits everywhere it's likely to be aluminium. Also steel & aluminium tend to be springy - if you try coiling the bare metal it will resist, whereas copper will retain the bend. Granted it's a bit of a destructive test - you could always try measuring data transfer across it. I see a few reports that someone tested a know good 1M copper cable and managed 900Mb/s, they only managed 300Mb/s with this cable - if it was copper I would expect to see very little differences, certainly nothing that extreme!

25m and if cat5 might only do 100mbps. Anyone knows?

This is almost certainly CCA so NOT Cat5/6/7.

This isn't bad if you need cat6. Amazonbasics have a bunch of cat7 for a good price also. I use the cat7 and they are fantastic.

10gb cat7 vs 1gb cat6, not that most people will need that sort of speed. Cat6 is the minimum people should be buying these days.