The rules for the Helilanes require visibility of 1000m, or 2000m in the vicinity of Heathrow.
Under IFR some of the routes could not be made available to any aircraft because of low maximum height rules. H3, H9 and H10 all have sections lower than 1000ft, which is OK under SVFR but not under IFR. Unfortunately, these are some of the more commonly used routes.
The thing to bear in mind is that the London Special VFR rules were introduced to increase the flexibility afforded to helicopters. Under IFR, this flexibility would be reduced.
For example, in ideal conditions (sufficient manning and reasonable workload) ATC request just 3 minutes notice by R/T before zone entry in order to issue a clearance. It is often totally impractical for a helicopter pilot to put in a written flight plan because of the type of ops involved (e.g. no facilities and no-notice changes to the task) so this is of huge benefit.
Also, under SVFR (and 6kms visibility) a visual separation can be offered to pilots of opposite way aircraft on the routes; in practice the right hand rule is often used for aircraft to pass each other. Under IFR this would not be allowed, standard separation would be required.
