Normally, you can use English at Dieppe. Don't know when you are planning to fly there, but you might be interested in the message below from Gerard Stephan, the AFIS officer.
From: Direction aeroport <direction@...>
Subject: Re: Arrivals Friday 7 October
To: xxxxxx
Date: Saturday, 10 September, 2011, 9:45
Mr xxxxxx, Bonjour.
Unfortunately, I will be not in Dieppe, on 7th Oct.
The tower will be closed from the 5th to the end of october.
But, if you decide to visit Dieppe on 7th oct. it would be better to send me a mail at least the 4th before noon.
The details I need are followings:
Type and registration
name of captain
POB
Date and ETA Dieppe/ Departure field
Date and ETD Dieppe / Destination field
The only problemwill be to refuel if nobody is in the aeroclub.
Best Regards
Gérard STEPHAN
You also have to leave France from a Customs airfield, so cannot fly direct from St Cyr to the UK.