To be fair, if a cable's maximum current-carrying-capacity is 25A (in ideal conditions), then surely it is accurate to state that it is a 25A cable.
If it is required to carry 25A in a situation where 50% derating must be applied, then it will be necessary to find a 50A cable.
How else could manufacturers and sellers indicate the current a cable can carry?
The safe way would be to quote the minimum CCC, the only other valid way would be to quote a range (e.g. "12.74A to 28.5A depending on application"), or to provide a table. But IMO, the one way not to do it is to give a single CCC (e.g. 25A) which is misleading and dangerous as it gives zero clue to an uninformed user that it might not be 25A (could be getting close to only half of that).
I know there's an argument that someone who doesn't know how to select the right cable for an application shouldn't be selecting a cable. But people do, for example, buy a piece of cable that's been stated as having a single current rating and assuming that it will be suitable for carrying that current. It's more likely to be an issue with smaller sizes (e.g. thread a piece of flex through <somewhere>, stick a plug on one end and a socket on the other end, run a tumble dryer and washing machine off it - as I found in the outhouse when I moved in with my now wife, her late father was a bit of a BIYer
).
Lets say the user chooses a 1.25mm² flex because it's rated at "13A", threads it through somewhere, and sticks a plug and socket on it. Is it suitable to be protected by a BS1362 13A fuse ? Tricky to answer as table 4D2A doesn't list 1.25mm² - so as an approximation you'd have to interpolate.
Also, BS 7671 doesn't give a rating factor (note that they are called rating factors, not
derating factors) for a BS1362 fuse. But looking at table 41.4 the maximum Zs for a 13A BS1362 fuse is the same as a 20A BS3036 - and the other tables in that section have similar equivalences if you interpolate. On that basis, the rating factor for a BS1362 fuse would be lower than that of a BS3036 fuse - i.e. less than 0.725. Just interpolating table 4D2A and applying rating factors of 0.95 and 0.725 gives a minimum current rating of around 8.6A for ref method A, clipped direct it's around 11.9A. In reality they will be lower still, but I don't have a rating factor for a BS1362 fuse.
And yes, the cable might be in insulation. When I bought our rental flat, it came with an extension lead that went from one of the sockets above the worktop, through a hole into the space behind the cupboards, and worked it's way to where they wanted the fridge. And that bit of space behind the cupboard was stuffed with insulation (not sure what for !).
And I'd argue that it's not "derating". To say that it's "de rated" suggests that there is a higher "true number". Any single figure you pick will be a "rating for this set of conditions" - choosing different conditions doesn't mean you are lowering the rating, you are simply choosing a different entry from a set of sets of conditions.
It would be equally valid to say it's rating is 12.74A, and depending on usage it can be uprated to various higher values.
But as for the OPs question, there's little doubt that if he wants a 7kW charger then he (or his electrician) will be pulling in a new cable.
