What I'm told by some quite senior people at the Met Office is that at present the state of science and technology available, combined with the nature of British/European weather mean that it's not possible to forecast with any accuracy beyond 5 days.
The UK Met Office have an ongoing research objective of extending their forecasting capability by another day up to 6 days. They're throwing millions at this, and hope to be able to achieve this within a few years but, at the moment, won't give a timescale for this.
In that perspective, I'd not trust a 5-10 day forecast for the UK or surrounding countries as being useful at any level beyond climatic means - which doesn't really need a forecast.
It's a bugger isn't it!
G