Berkley whiplash crystal all the way, i use 45, 50 and 55lb across my favourite regular multipliers for UK fishing. Its probably as thin as 30lb powerpro or others, but with higher b/s, more supple and makes great braid to fluoro connections.
I tried powerpro and daiwa depth hunter and didnt get on with them, took them off and replaced with whiplash crystal.
Shop around to find it for circa £25 for 300yds, typically enough for 1 reel with another 100yds+ nylon backing.
For (very) big game reels i use Jerry Brown hollow core from the USA, but its overkill for any UK requirements.