The date command has a number of switches, one of which is -d where you give it
the number of seconds since epoch, as in "date -d@1234".

This returns "Thu Jan  1 01:20:34 BST 1970" for the UK.

/etc/localtime is set to /usr/share/zoneinfo/Europe/London.

That's wrong, it should give "Thu Jan  1 00:20:34 1970 +0100 BST".

Additionally, you can get it to return as any string you want to, as in "date
-d@1234 "+%c %z %Z"" which should give "Thu Jan  1 01:20:34 1970 +0000 GMT".

After all, in January, the UK is not in daylight saving time.

However, what it does give you is "Thu Jan  1 01:20:34 1970 +0100 BST".

It therefore gives you the current daylight saving time status, rather than
that requested.

I assume currently, this will give erroneous results for any requests in
daylight saving.