My recollection of policing incidents involving firearms officers that made the news - is that it was certainly not commonplace. Armed policing was contentious - we did not want to be 'like the USA'. 'White House Farm' occurs two years after WPC Yvonne Fletcher (Libyan Embassy siege) and two years prior to Michael Ryan (Hungerford shooting spree) - both very high profile incidents in the news.
I've read somewhere that the issuing of firearms to police required a number of pre-conditions to be confirmed - but cant seem to find any further information. It would also be interesting to know how many incidents occurred in 1980's involving armed police. I'm sure there would have been other 'lower profile' incidents that perhaps didn't make the news. I wonder how many firearms incidents occurred for Essex Constabulary in the 1980's?