For those of you who have never been involved in the staff work associated with deciding if a risk is ALARP (and judging by some of the comments in this thread it is a fair few of you), one of the [many] things that is considered is cost. The financial cost of fitting TCAS to an operational FJ platform is not a simple as it might be when pricing fitting TCAS to a large ac....there isn't much avionoic space left on a FJ...so what gives to make way for TCAS or a CWS. Moreover, and Alfred The Great is the only one to mention it, what would be the "cost" of training [Tornado] FJ crews to operate with TCAS in peacetime only to find that it was not useable during operations against an hostile force? Would crews forego sound practices such as look out, use of current on-board systems and operational airmanship in favour of TCAS?
Let's not forget 2 things: the only way to eliminate mid-air collisions is not to fly at all (or only fly one aircraft at a time); and we train RAF FJ crews to operate in worst case operational conditions.