Overlapping Areas of Cover in Uncontrolled Airspace
 

I hear that an aircraft flying VFR below the London TMA could be speaking to any one of several different air traffic control units, neither unit having much idea what the other is doing.

This is quite different to flight within controlled airspace where there are clearly defined sectors, and you can generally work out who's talking to who based on where the plane is.

So when there's several aircraft in the same area all speaking to different units, does this cause problems with providing traffic information and advisory collision avoidance vectors?

If not, why not?

Unit specific transponder codes will be used to enable each radar unit to see who is working which aircraft, thereby enabling co-ordination to take place.
Farnborough will use codes beginning '04xx' or '50xx' for instance, and Heathrow/Thames Radar will use codes beginning '703x'.

What makes it even more fun is that the aircraft don't need to be talking to any unit.

After all, it's Class G airspace.