Which airport / terminal.
Average price is £30-£35 pp per session.
not usually anything massive floating round discount wise, particularly if travelling at busier times of year.
You can get a membership that gets you some free/discounted entries, but the card costs so no use for a single trip
The like of holiday extras may give a small discount
You might get some cashback if you use those sites Topcashback etc.
Pre-booking is generally cheaper than walk-up, though I got a deal from somebody at John Lennon doorstepping me for £24 as I walked past the door as they were empty in January.
Failing that, ask Traf