I live nearby and go semi-regularly.
Definitely do not pay on the gate prices ever, there's always deals online/on food packets etc so save yourself some money.
Get there before opening time (20 mins or so) so that you have time to park and walk up to the gate as it opens. The car park is pretty close to the entrance so no space is more than a 10 minute walk from the gate. Get on any open rides while its quieter and you won't need a fast pass.
Go to poundland or any other cheap shop before you go and buy 2 for a £1 ponchos for your group. Do not go on Tidal Wave without one, you get soaked through to your knickers. Great ride though.
Brace your head back when on Nemesis, you can get bumped around a bit. The corkscrew loops are worth it though.
Detonator is much more thrilling than it looks and normally doesn't have too long a queue even when it's busy.
If people are queuing up outside of X and being held back by a member of staff, it usually means the queue inside is fairly short. They cheekily leave the queue time as 40 minutes or so at the end of the day but it is normally much quicker as its quieter (queue) inside (not quiet music wise, it's like a rave in there).
You can rent a locker, non-refundable , I think £1 if you want to leave bag/water/lunch for the morning save carrying it around. Lockers are near Storm Surge and customer services.
I will come back if I think of anything else.