I guess we'll have to choose, but it seems a shame that I have to narrow our options to 1 school purely because two neighbouring counties can't put their testing on different days!
Unfortunately there are bound to be clashes of testing when the window for 11+ testing is so small. However, you don't have to exclude Bucks from your plans - from the Secondary Admissions Guide:
It is strongly recommended that the Preparation Test is taken before sitting the Transfer Test but it is not essential.
There is a familiarisation paper available to download from the BCC website which will at least give your son a chance to see the format of the test.