Oulton Park is really great.
Donnington is fast and flowing and generally suits road cars with 250HP upwards or Caterhams with 150HP upwards.
Oulton Park has a lot more tighter corner so is not as fast or flowing but is fun in a different way, though it suits smaller cars better it still remains fun in a bigger heavier car. But you can fun at Oulton in say a 100HP Caterham or a 150HP road car, but I'd say its harder on the brakes due to lower speed tighter corners and there been a few of them so do make sure your brakes are more than upto the task.
I've done Bedford, Silverstone, Anglesey but never find myself wanting to return there, but Donnington and Oulton are always calling, only other track I totally love is Spa, but its a bit more than an hours drive away and Spa definetely suits cars with 300HP upwards or the proper fast Caterhams as though it has some tight corners its got epic straights and some really fast corners too where horsepower is important, again strong brakes are required as in a car with some power you braking hard from 150-180mph on the back straight.