You should be fine. So long as there isn't a technical problem with the aircraft then you should be OK. (And if there is a technical problem CBG is THE place to have it!). Don't get too early to Cambridge, but there is a lounge (some photos in the lounge thread, and also here):
http://www.flyertalk.com/forum/briti...l#post25256312
The CBG service decants directly into non Schengen area in GOT, so unless there's something unusual you'll be able to go off to LHR easily with no security or passport check. If you are on time, you may want to clear passports since the GOT lounge is in the Schengen area, and it's 2 minutes from passports. So all being well you should have well over 30 minutes in the GOT lounge (which is a shared facility, nothing special, but it will do you a coffee and salad).
In short, I see no problem.