Hi Celia,
To answer your question regarding the Chesham vs Great Missenden starting with commute times Chesham is 60 minutes and Great Missenden is 46 minutes into Marylebone then 5 minutes on the Bakerloo line to Oxford Circus.
We have moved many families to Great Missenden vicinity and the HS2 question is very much the hot topic. The people who buy are aware there will be some impact with regards to the construction and the eventual train noise when/if it is built. So many people move out of London and are familiar with train or tube noise so therir acceptance is higher than others. There are many families who have lived in the Chesham for years and years and would not consider moving as they love being close the fantastic NT Ashridge countryside on their doorstep.
Chesham is more of a town so more cosmopolitan with a variety of eateries, shops and Waitrose and Sainsbury’s supermarkets.
Great Missenden is more of large village with pubs, cafes, one supermarket and lots of lovely villages nearby for walks and pubs etc.
We have a free area guides on both places that you might find helpful
Chesham
http://www.chilternrelocation.com/chesham-area-guide/
Great Missenden
http://www.chilternrelocation.com/great-missenden-area-guide/
I hope you find this helpful, good luck with your search Sharon
Chiltern Relocation