The weather diversion scheme seems to be interpreted differently by different airfields. Last summer, a friend and were at Manston, where we'd stayed overnight due to thunderstorms, trying to get to Le Touquet. We waited for the mist and low cloud to clear, then left, thinking we could get above the low cloud, and knowing Le Touquet was clear. With the cloud building, we eventually dived below it, and decided to divert to Lydd, where the weather was marginally better than at Manston, and anyway, we were tired of Manston. We were quite amazed when Lydd said it was a weather diversion and waived the landing fee. We wouldn't even have thought of asking under those circumsances, since we could have gone back to Manston.
It would be quite nice if someone would standardise this whole issue, wouldn't it.