Given your potential choices I think you will find this forum has a wealth of information.
For Parmiters and Watford Grammar, look under the "Herts" forum, and you will find leads and information for the entrance exams for the two schools.
For Bucks you will find much more help on DGS and CHS if you watch the "Bucks" forum from time to time. If you are inside the Bucks catchment area, then DGS is an option. If you are not, then only Chesham High is really a possibility. Places are allocated on "distance from school gates", and all the places at DGS are given to children who live in Bucks. Chesham High offered places this year to "all", i.e. all applicants who passed the 11+ and had applied for the school, regardless of distance from school gates.
I can only give a brief summary of the nature of the Bucks schools - DGS is a "Specialist Science Academy", and CHS tends (only tends!) to be the more arts-based school. Both achieve excellent results. DGS is the more academic of the two, CHS is the more inclusive, according to hearsay.
Do post back on either "county forum" for more help.