Welcome to the forum. The South East is amazing for Airsoft sites, especially if you are willing/able to drive. I've been playing for just under 2 years and am still nowhere near checking out all the sites within a 2 hour drive(i'm based close to Xsite Lane End)
Lane End is my local
Xsite's other site The Outpost is a fun CQB site
The Mall in Reading is good for CQB
Absolute Airsoft doesn't have a very good reputation, I would avoid purely due to it's affiliation with Special Airsoft Supplies(I haven't been but know people who have and said it's crap)
Rift Com3 is a good woodland site, players and marshals are sound
UCAP: Green Ops is woodland but CQB mostly due to the layout, enjoyed the site but the players on my team that day sucked
UCAP: The Sandpit is a massive site, I think it is also massively overrated. I wouldn't go again.
Zed Adventures: Battle Lakes, big woodland site that I really enjoyed.
Bluestreak Airsoft, like a lot of sites gets very mixed reviews. Thought the site was very well run, no issues with hit taking whatsoever and the lunch was really nice
Hopefully that gives you a good starting point.