Actually they had overnight counts in 1992! Gerry Adams losing West Belfast happened in the middle of the night. Don't think any happened overnight for the next few years.
Indeed, I was going to say that, because it's in the BBC coverage, as repeated the other week - and makes the last few hours a bit dull because everything's done and dusted by lunchtime and there aren't even many Northern Irish results to cover. I think only a few seats counted overnight, possibly just the Belfast ones, with the rest counting during the day as was familiar at the time. I think it was an experiment they didn't continue.
There used to be quite a few seats across the UK that didn't declare until Friday daytime. In 1983 Alan Beith is in the Beeb studio virtually all night because his seat in Berwick doesn't declare until the next day, and in 1997 I remember watching William Hague's declaration in a politics lecture (Jeremy Vine said that period was the best time to be a Political Correspondent as the government was in total chaos, and it was a great time to study it too). In 2001 I remember the last seat in England to declare was the Isle of Wight and it was only on ITV as the Beeb finished at one but ITV came back for an hour at two o'clock and caught it.
And in 1987, the Truro By-Election declaration was on Friday morning and shown on Open Air - http://genome.ch.bbc.co.uk/schedules/bbcone/london/1987-03-13#at-11.35