Interestingly, from what data I've been able to find (a study by John Curtice of second preferences expressed on the British Election Study), AV would most likely have inflated the majorities in 1983 and 1997. The main finding was an increase in Alliance/Lib Dem seats, but only in the 1983 election would the Alliance have been within spitting distance of Labour. AV seems to keep the two-party system intact, but make a number of less marginal seats more marginal.