I wouldn't import one unless the rules are less restrictive for an older car. GM has specific euro-export models with a number of changes, including a rear fog light (req for England) and amber turn signals (req for elsewhere in Europe, not sure about UK, and which require a decent amount of rewiring). I have a friend who recently considered importing an Audi AllRoad from the US to the Azores and gave up because of all the red tape and changes, and this was a European-design car to begin with.
I would also not buy an 84 because there were a lot of one-year-only parts that may prove difficult to get if needed. Best mileage, best reliability can be found on the later C4's (93+, 92 was first year of LT1 and less reliable) but they're also harder to work on (less room under the hood) and more expensive so it's a tradeoff.
If you like DIY car work, I'd consider the later L98 C4's (88-91) because they're cheaper, parts are easier to come by, they can be tuned for a lot of power, and the 6-speed version still has decent gas mileage. Given England's, um, DAMP climate, you also probably want one with ABS, which rules out anything before 86.
Good luck with the search!
[RICHR]