Does it have to be holographic?
For that budget (£300-£400 end) you should be able to get a nv 552 eotech or an aimpoint like an m2 second hand.
Personally though I think they're overkill for airsoft, even if you do events on a regular basis.
I'd recommend vortex, the cross fire red dot is good and versatile, their warranty is apparently awesome (not had to use) and you won't be able to get repairs on a legit eotech or aimpoint (at least not easily). Holosun do some great options as well. For these you're looking £150-200 ish. They're for real rifles so recoil won't be an issue, they have a warranty and NV modes ( if that's important). Tactical kit is one of many UK retailers.
The holy warrior clones are the best clones I've seen, good on bright days, have NV modes and seem to work well. NV modes can be a little bright (based on the eotech XPS I have). Tactical waffles is the only UK reseller I know of, devt team six sell them from Europe or you can get them direct from Ali express but post takes a few weeks. Cheaper, good enough but not perfect.
