As you're going to a destination within the EU, there is no requirement for you to notify anybody at your outbound airfield (other than for your flight plan). The French insist (since you are will be arriving from a non-Schengen country) that you arrive first at a customs airfield (which Le2k is).
For the return, you must again leave via a customs airfield (no problem again with Le2k), but if your arrival in the UK is to a non-customs airfield, you must comply with the local procedures. What these are can probably be best obtained from the airfield manager, or failing that, from C & E (suggest you ask the C & E at the nearest customs approved airport, preferably in the same county. If this fails, there are more numbers in the AIP)
FWIW, at White Waltham, we have to give a minimum 4 hours notice of our return time, which in practice means for a lunchtime jolly over, we let them know as we file the outbound flight plan. Should we arrive back before the declared time, we are supposed to sit *in* the aircraft until either the declared time passes, or we are visited by the friendly Customs people.
Something else you may want to consider - Le2k sometimes seem to have a bit of a habit for 'losing' flight plans filed there. Suggest you file both the outbound and inbound in the UK before you depart, if you are VFR, the French are very laid back about your declared and actual 'off chocks' time
hth
IFR