Its a problem all over the UK to be honest, runways are often reported wet when they are damp or even bone dry as someone said above. Common sense and a look out of the window are often much more reliable.
If its a landing then you just have to take the report at its word but at least you are erring on the caution, wet landing distance being longer anyway. The problem is the departure, there is I think an assumption by many (airport operators, and a lot of pilots) that it doesn't matter if you use wet performance when you don't need it, you are covered anyway. This is most certainly NOT the case, wet runway performance is a fudge to allow operations to be commercially viable (ie. lift a decent payload) during wet conditions. It provides significantly reduced obstacle clearance margins (15ft rather than 35ft) if an engine fails at V1 and less margin if you have to stop before it due to reverse thrust on the remaining engines being factored in which it isn't on a dry runway.
The regulations are very specific as to what constitutes a dry, damp or wet runway and its about time the airport operators worked to them and provided regular and accurate updates to the runway state after it has stopped raining.