We live nearby and have merlin passes so go a few times a year. There is more to it than just the rides but you can miss it - there are at least 2 creative areas where you can build models, shows, the actual miniland bit (which is fab - lego models of different countries etc) and a small water park area with fountains, water cannons etc and 2 or 3 adventure playground type places, slides etc. I think for the distance you are coming you need to do 2 days at least. If doing 2 days it is possibly worth considering getting standard Merlin passes when they are on 20% off days, normally around the Easter and May bank holidays. You can generally get them for about £90 odd per person and then you can go twice and also to chessington or Thorpe Park as others have said if you have time, or even London Eye and London Aquarium in London. Plus 10% off in the shops. Chessingtone is good for younger children and has an aquarium and zoo.
I think drive too then you have the flexibility to go into London, Windsor, other theme parks etc - would cost a packet on public transport. Isn't near any thing relaly and you might want to drive to get food etc.
It is hellish when busy - I really wouldn't consider July/August when the English schools have broken up, or if you do, go mid week, not weekends. If you get a good day busy, but not rammed, it is a great day out, we love it. We never get Q bots as they are insanely expensive imo, but go less busy times instead, or try to. I went during the Royal Wedding and the last England World Cup game and it was great! Almost no queueing (none at all for a couple of hours during the football). Not sure you can replicate that but I wonder if actually during the Olympics might not be a bad idea if you are staying on site in the Hotel and don't have to worry about traffic - might be quiet during the opening ceremony or the 100m final!