The relevant case, Honourable Society of the Middle Temple v. Lloyds Bank governs the status of cheques, whatever terminology you care to use. You may well call them "negotiable instruments" relying on outdated definitions, but that won't help you in court when you claim to be a BFP (bona fide purchaser for value, or holder in due course) of a UK cheque, and you want to sue the issuer on it.
The full judgment is at:
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In other words: even if they are "negotiable instruments" they aren't negotiable.
Or so the judge in my case ruled. (The cheque was, in fact, payment for damages to my car by the driver of a Polish-registered vehicle. I was no more successful in getting the Polish insurance company to pay (even though I was rear-ended) than I was in collecting on the cheque, written by the English passenger (a well-known actress, BTW) who had hired the driver to move her stuff (although I couldn't prove the employment relationship in court, and she denied it then).)
The problem for non-lawyers (especially those in common-law systems) who would argue issues of law (and definitions such as "negotiable instrument" and "cheque" is that things don't mean what the dictionary
-- or even the law -- says they do. They mean exactly what the House of Lords and the European Court of Justice say they do, nothing more and nothing less, in England at least. So if a "negotiable instrument" isn't in fact negotiable, I as a lawyer run out of words when I try to explain it to a non-lawyer.
And those non-lawyers who argue it among themselves will only confuse each other.
The operative instrument is "stare decisis".
Note that modern legislative drafters (and there are currently 42 of them working in Parliament) try to define words succinctly at the beginning of legislation to tie the hands of the judges. They don't always succeed, and sometimes they insert more doubt and ambiguity than they remove.
I add that in Middle Temple "The cheque was crossed 'not negotiable a/c payee only'". Perhaps an open bearer cheque could be written, or a crossed cheque could be un-crossed. But that virtually never happens. And a cheque can be bounced months after it has been written: in the UK there is no way to make a definitive payment by cheque. You have to use a banker's draft or a wire transfer (BACS or SWIFT). This is reality, and pedantry will not be useful to the person who gets scammed on eBay and only finds out months later that the cheque he deposited was countermanded. (I leave aside the issue of bogus cheques and the whole issue of preferential payments and their recovery in bankruptcy...) Funny how on these ngs the amateurs flame the professionals.