Budget is £120-£150 doubt I’d get a down one for that , it’s for bikepacking and needs to keep me warm down to -2 ish.
You probably would get down for that. I did, but I'm a pretty warm sleeper.
e.g. Eurohike
https://www.ultimateoutdoors.com/equipment/133692-eurohike-down-500-4-season-mummy-sleeping-bag.html
+ some clothes might be good enough...
Mountain Hardwear Hyper Lamina Spark is somewhere around the top-end of that price range and about as good as it gets for a synthetic bag. If you want down, the Alpkit SkyHigh 500 is decent at £169.
Alpkit.
Yes cheers might push the boat out for the Alpkit one
Same question earlier this year and went for a RAB Neutrino 200 on sale at Bike24, they also had the Neutrino 400 as well. Both really good for warmth v weight
Cheers will check out bike24