In the time that I have been following the Met Office seasonal forecasts, IMO, they have been generally very good. If I remember correctly, they didn't do too well forecasting Summer 2003 where I think they went for an average Summer, needless to say what the actual outcome was! Also, their forecast for Summer 2007 was off the mark too. And of course they called this Winter wrong in September and October. However, their general forecasting for the Winter season is pretty good and they also state that they have higher accuracy for their Winter forecasting. Indeed, apart from Winter 2008/09, they have got all Winter's since 2001 pretty much spot on.
On balance I would conclude that they are not perfect, but pretty good.