The unfortunate thing is, the location of most of our parks prevents them having a super [Disneysque] style motorway or traffic road, which is purely there just to specfically cater for theme park goers.
Instead we've pretty much got single carriage roads towards all the parks. Towers is based through long windy countryside roads, aproximately over 10 miles from any main road. For newbies, they could almost argue whether the Uk's 'supposed' biggest theme park is located there. Legoland suffers from being in the middle of a residential area, natural area and also being close distance to Ascot and Windsor castle too. If the park could receive similar lane infastructure to their park drive [2 lanes each way], the road would flow significantly better.
Chessington is positioned slapbang in the middle between the motorway and another large residential area and like the other two above, road expansion doesn't look much of an option, unless all those pubs, houses and businesses decided to sell up.
Thorpe has the lesser problems here, with it using slightly better infrastructure on the roads, but there is still room for improvement and from time to time, traffic still builds up.