> Merriam-Webster has it as "chiefly British spelling" of jail
It's odd that it doesn't mention it as archaic, because it's provably Just Not Used in the real world [0], but it's also an American dictionary, so all bets are off
If you click on the entry your search results give, it takes you to the entry for jail (eg: jail is the canonical noun it's using). Jail and gaol have slightly different routes into English, with gaol being via Northern French. Sez the OED:
> "remains as a written form in the archaic spelling gaol (chiefly due to statutory and official tradition); but this is obsolete in the spoken language, where the surviving word is jail, repr. Old Parisian French and Middle English jaiole, jaile. Hence though both forms gaol, jail, are still written, only the latter is spoken. In U.S. jail is the official spelling."
Finally, though, "jail" as a noun is pretty infrequent compared to "prison", which are the same thing in the UK (unlike the US), but the latter is much more common. The verb form on the other hand is almost always "jail", so when you are "jailed", you're sent to a prison.
> I believe Collins is the standard dictionary that British people use
And you would be wrong. They've only been putting out a dictionary since 1979. OED[0] is The Dictionary in the UK, and I've quoted its take on it in an adjacent comment.
Well, come on, that's obviously false. Zero percent of British people own an OED or even have the resources necessary to consult one when they want to look something up. It costs over a thousand US dollars.
I suspect fewer than 1% of British people own any paper English-language dictionary, but the sort of people who need a dictionary beyond what dictionary.com can offer will have online access through their employer, their institution, their local UK public library, or for the princely sum of £8/m for a personal account when paying annually.
Regardless, the authority that the OED holds on British English is best understood through the criticism section of it on Wikipedia, with people decrying its absolutely overwhelming influence:
> criticizing the OED is extremely difficult because "one is dealing not just with a dictionary but with a national institution", one that "has become, like the English monarchy, virtually immune from criticism in principle"
Many libraries in the UK and Commonwealth countries have the full volume set the first, second, or both editions of the OED.
I've routinely referenced it in both the university library and the local town library since 1980.
There's been a digital version of almost all of the second edition kicking about since just prior to the print release of the second edition - I have that on most computer images I own.
The OED offers digital subscription access to the second edition and to the in progress third edition which a good number of people, libraries, companies subscribe to.
It's the dictionary of reference for those serious about the English language.
I don't disagree that practically zero percent don't own a copy, but pretty much anyone in the UK can access the OED with a free library membership. It's pretty much the only thing I use my library card for these days.
If people do use a dictionary these days they most likely do use a Collins or Oxford Dictionaries (not the same as the OED). But I imagine that most people would just use a Google search and rely on the top box which itself is using Oxford Dictionaries for its definitions.
https://www.merriam-webster.com/dictionary/gaol
https://www.oed.com/search/dictionary/?q=gaol