Weather conditions are considered to be outside of the control of the airline and therefore an 'extraordinary circumstance'. Under the terms of R261 compensation is not payable for flight cancellations or delays due to weather. However the airline remains responsible for refund, reroute and care (meals and hotac) as applicable.
The weather conditions causing the delay are not necessarily just those at the passenger's departure airport. The airport of departure of the incoming aircraft could have been closed, while other airports (flown to by FR and LO perhaps) did not experience the same. The regulation does not define weather conditions, but it is not only 'freak' conditions.
The UK CAA is the NEB for the UK. By all means contact them and they will ask the airline for proof of the reasons for the delay. However if the flight wasn't to/from a U.K. airport expect them to put it to the bottom of the pile. Prepare to wait.