Revealing the answer to be 3:
Byron Dyce 3:57.34PR (and 1st ever sub4 of 3:59.6 on 16 May 71)
Gawain Guy 3:58.21 12 Jun 83 (subsequently switched to US and 3:57.18 in 1987)
Steve Green (as noted above, although he may have had UK citizenship at the time)
So, the upshot is that Campbell would be first Jamaican sub-4 miler in almost 20 years if he breaks the barrier soon...