The idea grew out of a recommendation in a CAA review of GA Accidents. The logic was that the thought of the landing fee might cause pilots to press on when they should divert.
Charles Strasser of AOPA took up the cause (you are a member aren't you?) as a result of which most airfields joined the scheme.
Personally If I had a genuine Wx diversion I would be so grateful to be safely down in one piece that I would't dream of quibbling if asked to pay.
By what you say you and others were en-route to Le Touquet and because of poor weather there diverted to Lydd.
If one's pre-flight Wx briefing indicated no cause for concern but there was a sudden and unexpected change to the weather that caused L2K to get socked in and one's return to one's departure aerodrome to become equally untenable with the result that one had no choice but to divert ot L2K it sounds like a genuine Wx diversion.
If on the other hand one decided to land at Lydd and see if the weather at L2K improved then it doesn't.