EWR to LGW, 198$ RT in January of 1987 (granted this was on People's Express, in the middle of the winter). I went to the UK several more times over the next few years and RT from the NY area to LGW was usually in the 200-300$ range.
These are USD. When I flew JFK to LHR this past June my flight was around 1600$, over 800$ of which was tax.