DFC
allocate a single squawk to FIS tracks... brilliant, but actually that's what many units do already? Check the SSR Code Allocation Table and you will find many units have their own conspicuity codes. These are used to distinguish to adjacent radar units traffic that whose SSR Mode C (height readout) is unverified... (i.e may be unreliable for height separation purposes)that's all, no guarantee that the track is either VFR or under FIS... but likely.
For myself, if I get a lot of traffic working close to our zone (or transiting through it) I will allocate individual squawks to maintain track identity... the pilot's not getting a radar service, but it speeds up/simplifies the process if/when I need co-ordination. We have a small code block... but manage OK, with multiple use of the conspicuity code when necessary... Funny how often I allocate this and another pilot will chirp up and say "I'm already squawking ****!).... well done for paying attention.
There are few rules for use of a unit's SSR Codes, and at many controllers use them at their discretion... What has always puzzled me is why there are so many separate codes for Police helicopters... I assume they rarely operate in the same airspace, are VFR/FIS most of the time... and one code is surely enough (like air ambulances & SAR helos have)?
The original question has been well asnwered... units have their own codes, there is at present no system in the UK to carry the codes forward from one unit to the next... but as someone has said that may be one of the advantages of Mode S?