If you register on the forum (bottom of the page, no spam or nasty stuff) you can send me - or anyone else - a PM and we can advise you in more detail, including things like commuting times!
The catchment schools for Beaconsfield are RGS and John Hampden at Wycombe - see map on this link:
Both are good schools, but you would be a long way off, and that would really limit your chances of a place at either. There might be places at Burnham Grammar, but as you can see from a recent thread on Bucks, that gets very mixed press indeed.
For Berkshire you definitely need to post on the Berkshire section - we in Bucks will only have the haziest idea of the admissions system there and which schools are worth going for.
One other problem (as if we hadn't given you enough to worry about already!) is that because the Bucks grammar system is so strong there are very few good private schools at senior level in the immediate area. Again, there may be some good ones in Berkshire (there's a nice one right by Windsor Castle, I believe?
) but they may also not be an option because of cost/high entrance standards.
One private school worth a look is Clare's Court School near Maidenhead. Not a high pressure school, but a very good all-round school with a lovely atmosphere. All the children I have known go there have been incredibly happy.