Weather - Of coure this will be your first stop as nothing happens unless it is suitable
NATS AIS - The UK AIP/AIC/Notams - access needs free registration (never understood why)
Skydemon Light - A free VFR flight planner for UK airspace sponsored and approved by NATS
French SIA - The French AIP (no need to log in and NOTAMS are easier to use).
French VAC - This is where to get French airfield information and plates