Like others have said, it really depends on what you want for your twins.
My two children have both been at Aldenham prep school. The older one went all the way through to the end of Year 6; less confident and quieter - it was the perfect nurturing environment at the time.
I withdrew my younger child earlier this year for two reasons (1) a noticeable drop in academic standards under the new head (probably due to the exodus of experienced teachers) and (2) to move to a larger school which is more dynamic, has lots of sporting and musical stuff going on (not possible in a small school because there aren't enough children) and which is much more about fostering independence.
I would suggest you think about the following:-
Aldenham school is very small (just one class per year group) so your twins will be together all the time until the end of Year 6. On the plus side, the small size of the school means that it is warm, nurturing and friendly. However, it also means that it can feel claustrophobic and overprotective, especially as they get older.
It is not an academically selective school and it considers itself the Prep school for Aldenham senior school only. Therefore, those who are interested in moving their children to academically selective senior schools (e.g. the local grammars, Habs, MTs, NLCS etc) are paying for tutors to ensure their children reach the necessary standards.
If you have a decent state school option near you as a back-up, then I would suggest that you turn down Aldenham and save yourself all that cash.
Good luck with your choices.