What seems to be missing from the discussion here is the "odd" way in which the UK handles the IR/IMC rating, the difference between IFR and VFR and the consequences of all this.
In most of the rest of the world, the distinction is quite clear. You are either VFR or IFR. If VFR, you are (supposed to be) in VMC (what VMC is depends on the airspace, obviously) and you can do whatever you please but you are responsible for your own separation (in most classes of airspace). If you are IFR, you have filed a flightplan, which you follow, you have an IR and ATC is responsible for your separation (in most classes of airspace). Very clear-cut.
With the UK IMC rating the difference between VFR and IFR becomes less of a formal thing, and more of a state of mind thing, leading to situations where a pilot can instantaneously (without telling anyone) switch from VFR to IFR and back in a lot of situations, to suit whatever is best for him. Can't get an IFR clearance through controlled airspace? Well, he'll take the VFR clearance then. But since he's got the IMC rating he'll possibly stay less clear of cloud as he's supposed to.
Mind you, the IMC rating is a fantastic thing if you look at the current (theory) requirements for the PPL/IR. But we all have to accept (plain PPL, PPL/IMC and PPL/IR) that there are pilots in the sky (over the UK) that are flying with an IFR mindset, but rely on "big sky" for separation. And depending on the actual weather, they may be in the same sky as a plain VMC/VFR PPL (but with their heads inside the cockpit) or in the same sky as a PPL/IR on an IFR flightplan (but without talking to ATC or having filed & following a formal flightplan).
So VFR pilots relying on see and avoid for separation may encounter other planes whose pilots have their heads firmly in the cockpit (because in their mind they're flying IFR), and IFR pilots may have it the other way around.
It's therefore not just the IMC pilots who accept that their separation is largely provided by the "big sky", but they place others in that same situation. Looks like that's what happened here.