It's a built-in variable in webCoRE that keeps track of the offset between your local time and GMT in coded time format. So during your standard time (GMT) it will be 0, and during your summer time (BST) it will be some number.
I'm in US PST time zone, and if I enter "formatDuration($tzOffset)" into a webCoRE evaluation console below the piston, I get: "ยปยปยป (string) -08:00:00", I am currently 8 hrs behind your GMT here. During my summer time (PDT), I jump forward an hr (closer to GMT), and this expression should give me -07:00:00 (7 hrs behind GMT).
I think yours should change from 0 to +1 hr when you change from GMT to BST. It should give you 3600000 (integer ~ milliseconds) during BST and 0 during GMT in datetime format, if you don't use a function to convert it to a more readable time.
In any webCoRE condition you can select expression, and enter formatDuration($tzOffset) as part of your expression to get the text string time version, or just select variable and get the integer version. You can always play around with your expressions in evaluation console to see what works.
Hope it helps. Cheers!
BTW, might get more eyes on this specific type of query regarding webCoRE at:
(at least until HE fully assimilates it into this forum).