My understanding is that if you're working in tech at GCHQ, you're dealing with fossils—both your colleagues and the actual tech stack itself. Apparently any kind of meaningful change has to go through countless layers of scrutiny and review, taking weeks.
You also need to basically nuke your social media and lie to your friends and family on the regular. Which IMO effectively means you need to be thinking about your job 24/7, and therefore are working 24/7.
~£40k isn't even close to the low-water mark for an entry level job given all that IMO. If they want such a specialist skillset too, they're probably gonna need to add a zero if they actually want to attract anyone good.
There are even other parts of the government that don't have all that baggage and pay more.