The TriStar sims (-1 and -200/500) were not sold by BA to the MOD and indeed they were retained by BA until after they had sold off Caledonian to Inspirations in the mid 90s. Even then the MOD did not buy the BA sims but opted to buy a brand new one. The -1 sim went to BOH. I do not know what happened to the -200/500 sim.
The 6 -500's the MOD bought were a real bargain at the time £50 Million for 6. When later ex-Pan-Am jets were bought they cost £22Million each. Hence the controversy about fire sale prices.