Our legal system works in such a way that the exact legality, or otherwise, of certain actions aren't made clear until a test case comes before the courts. I doubt that the requirement to include instructions in English has ever been tested, so it's likely to be a grey area.
Electrical fittings don't necessarily have to be supplied with any instructions. The trade counter at an electrical wholesalers will supply plenty of items with no packaging or documentation whatsoever. Jewson's advertise themselves primarily as a trade supplier, so they might be safe to provide items with no (valid) instructions.
However, if an item is advertised as 'complete with instructions' and it's not provided with any (comprehensible) instructions, the vendor has clearly not fulfilled his contractual obligation to provide the item as offered for sale.
Similarly, if the purchaser could reasonably expect to be provided with instructions (in English), it could reasonably be argued that the item was not sold 'fit for purpose'. (As an analogy, suppose that somebody buys a new car and finds that the handbook is missing. He approaches the retailer who says "If you want a handbook, we'll have to order one. It will cost you �20". Under those circumstances, it's likely that a court would rule that the handbook should have been provided, free of charge, with the car as any reasonable purchaser would expect it to be included in the purchase, even if the vendor did not enter into any specific agreement to provide one).
As stated, the exact legality would have to be decided by a court of law but it would come down to deciding whether the purchaser could reasonably expect instructions to be included.
Chris