Some may remember the days before class A-G airspace, and will recall that Luton's airspace was a combined SRZ/CTR. What this meant was that it was IFR, only if you had an instrument rating.
I think your memory's playing up Donks.
Luton was never Rule 21 airspace (IFR only). The SRZ required contact with ATC, the CTR required a clearance if operating IFR. I think VFR minima in controlled airspace were more stringent (5 nm vis 1000 ft vertically from cloud) so it may be that the SVFR lane was used more often than now.
Or were you saying that a full IR was required for IFR, not just an IMC rating?