Longbow55
You don't need ATC clearance to descend "below clouds", here or in the USA.
In the USA, FAR something.something appears to prohibit an instrument approach other than a published one. In the UK there is no such rule (yet).
But if you happen to be able to do a visual approach from some level which you can get an IFR clearance to fly
at then obviously you can then land under VFR. You can call this a DIY instrument approach with a MSA of 1000ft AGL or more
Or you can call it "cancelling IFR". Same thing.
In the UK this is a bit of a non event anyway because in Class G you won't normally be receiving a radar control service so you can fly under own navigation as you wish. The only case I know of of a RCS in G is a low level letdown which is basically a radar vectored descent over a surveyed bit of ground, down to 1000ft AGL, but even then you can ask to switch to own navigation at any time.
The question then becomes: how low will you go? Flying
at the MSA (or your MOCA; I have the FAA IR too) gives you an IAP with a DH of 1000ft - not unreasonable over open flat country. Of course Americans have never discovered this
It all gets much more tricky if trying to get below say OVC005.