Evidence to back that up?
The UK Met Office is independently audited on its record and reliability (as are most Met offices).
Have a look e.g. in its annual reports for references. Typically it returns a 10-15 fold benefit to the country : crops, cost saved by knowing of incoming events, etc.
On seasonal and long-term forecasting, the problem is lack of input data, mostly, particularly ocean data (yes, models will be wrong, too, but unless you know what the ocean currents were, you can't compare observations and model to diagnose the model ...). This has been changing dramatically in the last 5-10 years with Argo floats, new satellites for salinity, sea surface height, ...
We can now do seasonal forecasting for certain areas of the globe 3-7 years out. Not Europe, yet, but we have good reason to believe it can be done. Seasonal and decadal forecasts have been visibly getting better year-on-year. I've more experience with the ECMWF and other European climate models that the recent HadGEM ones from UKMO, but they've all been improving, with increases in computing power enabling us to get certain features right.
When sources like the Daily Mail and Telegraph diss the UKMO because they don't like climate science, and then go on to predict -20 Novembers and blizzard headlines (which, didn't happen, not that they noticed), I tend to call BS.