Chilli, do you know why? Safety or just design of airspace so that you can fly almost anywhere IFR? Or on the other hand, is the UK different by allowing it?
Not unique in allowing it, but unique in making IFR in Class G airspace necessary for low-ish level IFR.
Most other countries have Class E airspace down to very low cruising levels, so you are "in the system" for the entire flight, although you are in an environment where other VFR traffic can fly pretty much the same way as in Class G [ok, higher weather minima, but that's it], so it's "see and avoid" for you...
In the UK, there is LOTS of completely uncontrolled Class G airspace, which is great for freedom, but poor for service... when you are leaving controlled airspace, you are temporarily "outside the system" and have to negotiate your way back into it.
For someone who is familiar with IFR (and ATS in general) outside controlled airspace in the UK, this is not much of a problem; but if you come IFR from another country, it can be a bit of a shock...