I would recommened going to Alton Towers and here is some tips to beat the queues. First things first, get to the park as early as you possibly can. I would also recommened booking your tickets in advance so you can get into the park 30 mins before the park opens to get XART (xtra added ride time) this means there will be no queues. Also if your early this gives you the option of getting the virtual queue tickets which im guessing you know what it is? If you dont, its a time giving to you to allow to go to 'X' ride at a set time and walk straight on without hardly queueing. After around 12 you will no longer be able to get the tickets. Around lunch will be the busiest time in the park. People will then tend to start leaving the park around 2. So if you plan to go later go around this time. The park then continues to empty as all the people that have been there in the morning usualy leave. Then again towards the end of the day the queues should be smaller but not a empty as in the mornings.
Other parks i recommend is of course Blackpool, however im not sure wether The Grand National will be operating by the end of August...touch and go. However i would still go for The Big Dipper and Valhalla. The park usualy has hardly any queues and is open to around 10:30 PM if you wish to ride in the dark. However Blackpool is quite a fair distance from other parks in the UK. Its about a 2.30 drive from Alton and around 4.30 from London
If you are around Alton Towers way, i would sugest you drop into Drayton Manor for a couple of hours. The park again hardly has any queues and Apocalypse is probaly the best ride in the UK. Also everytime i go on Apocalypse the queue is walk-on even on busy days. But is usualy only walk on, on the Stand Up and Floorless side. The sit down queue can get large. It also has shockwave Europes only stand-up coaster which can be fun along with Storm Force 10 and Malestrom.
Thorpe Park again is going to be very busy, due to it being in the heart of London. The queues at Thorpe can get really big and you will be looking at 2-3 hour queue for Colossus depending on how its running. Its a good park and if your around London way i would say give it a try
Also as said DO NOT go to fantasy island. It is one of the worst parks in the UK and is built into a market place
The entrance queues are hidden by people trying to sell you towels and second hand underwear and i would not go there. However the rides there are quite good. Ol Juibilee had a good first drop, but was rough the same as Millenium Coaster. They have opened Absoultly Insane there which looks fun, but not enough to sway my vote.
If you want any more information just contact me, and if you want more detail on Alton you know where to find me on msn [;)]