Wait, hang on. Are you getting 350fps with 0.25g BBs?
If so, that would be 1.42J, which would make that (technically) a Section 5 firearm, and way above any UK site limit for an auto-capable gun.
Specnas EDGE guns come with two springs, an M90 (which I assume is the lost one) and an M120 (which is consistent with ~1.4J)
If you've been playing with the M120, without your gun locked to semi and using DMR rules and a MED, I'd like to know what site(s) you've been playing at. And I'd love it if one of the UKAPA reps who frequent here could have a word with them about the wisdom of saying "350fps limit" and assuming that even well intentioned players have any idea what that actually means.
Anyway, tl;dr version, get an M100 spring and a chrono, or an M95 if you want to be more confident that you'll come in under the Joule limit for UK sites.