My experience was that we often didn't trust sharing alot of information with certain european partners because their Security Apparatus was often not at the operational maturity level we'd expect of a 21st Century Security Service in the post 9/11 world. There were also concerns of the political use of intelligence by certain jurisdictions to secure funding on the pre-text of "Hey we just stopped a Terrorist attack" when in reality it was a single guy informant / low level dude and a certain country jumped on the opportunity to "look tough on terrorism" and missed the chance to get a whole cell, or buggers another countries operation.
Intelligence is often wrong, sharing information that's often wrong isn't as beneficial as you would think, and it most certainly isn't "open sharing" as intelligence is often cherry picked, massaged and is never full disclosure.
The EUROPOL already co-operates with other non-EU countries, but be under no illusions the UK's Intelligence Apparatus as a whole, is the Jewel in the Crown of the UK Defence. Every country courts it, and wants its co-operation and help - EUROPOL will suffer if it ostracises the UK, and having seen whats happened in France and Belgium recently they need all the help they can get.