Going through some old papers yesterday I came across a survey I did during the first three months of 1997 of every 50 pence coin I got (presumably I must have got some from banks as well as in change). This was immediately before the older, large 50p coins were replaced by the current smaller ones.
Here it is:
1969 88
1970 12
1973 37 (EEC Commemorative)
1976 21
1977 27
1978 40
1979 15
1980 31
1981 20
1982 18
1983 21
1985 1
No 50p's were issued for circulation in 1971-2, 1975-5 or 1984. After 1985, no further Britannia reverse coins were struck for circulation. The EU commemorative 50p was issued in 1992 (mintage 109,000) and the D-Day commemorative in 1994 (mintage 6,705,200). I didn't find any of these in my survey.
1985 50p coins are quite scarce, with a mintage of 682,103.