I think it's unusual for /k/ to be completely replaced by /ʔ/, though it's very common for this to happen to /t/ in many European English accents (mainly in England and Scotland, but also parts of Ireland). Syllable-final /k/ and /p/ (and also /t/ in many accents where it is not replaced by /ʔ/) tend only to be pre-glottalised (or glottally reinforced), but this might be mistaken for a simple glottal stop by someone unaccustomed to hearing it. Native speakers do distinguish between /bæʔ/ (= "bat") and /bæʔk/ (= "back") (and indeed /bæʔp/ (= "bap")).

Just commenting so as to follow this interesting question, Benny. By the way, in American English, we never replace a K with a glottal stop, but I too have heard British speakers do so. You can hear final P, K, and T as a glottal stop in Cockney for sure (and related Australian accents, etc.), and maybe even in Estuary English. The glottal stop for T is much more widespread, occurring even in American English (in certain phonetic environments). This glottal stop pronunciation is, by the way, a useful way to notice where native speakers are making the syllable breaks….

Thank you for dropping by, Coligno. When I wrote <em>I'm fairly sure the /k/ is glottalised </em>I was hoping that an expert would confirm or not.

So the /k/ in "Back then" is usually pre-glottalised or glottally reinforced? Does this mean that it's a /k/ with a glottal quality? Or an (almost) glottal consonant with a k-like quality? But not a 'full' stop?

Are we speaking about this glottal stop but on the on the word back.

The same as on the stereotypical cockney "butter" OR "Mountain"

In that case I as a Londoner/cockney would say you all have an antiquated view of the cockney speaking.

I don't think I would ever say back or back then in the same manner as saying "butter" in cockney or "mountain" in cockney and English, there would be a release to emphasise the --cK but not a very sharp sudden stop as in the video for "butter" and "mountain" that would be extreme even for a cockney of old, that Mr Van Dyke got so very wrong.

In my opinion even an old cockney speaking the more stereotypical cockney would have a speech impediment, if they said BA -----silence --- C--KKk, whether by itself preceding a word or after another word, such as "back then".

P.S POST SCRIPT late Edit.

I think you may have misheard and misunderstood a full glottal stop with a glottal-ly reinforced sound as explained by @ Colingo, note I am not a linguist.

Yes, I think it's very common. When we say something like 'Back then', I'm fairly sure the /k/ is glottalised. Most of us do this, but few of us are aware of it.