The obvious alternative is to "buy abroad"...meaning, in all likelihood, from the Americans. The trouble with this is that all the money spent is lost to the UK economy; at least, buying from national sources means that the money stays in the UK and gets disseminated as jobs, and can be reinvested. The industries concerned retain their capabilities and the skill set of their workers. Restricting the argument to the horrendous cost doesn't give the whole story...and remember the F-111 we were going to buy? Boeing's SST cost more than Concorde and all they got was a plywood mockup...
I agree that part of the problem is "moving goalposts"...and we surely need better contract supervision and purchasing control...but "state of the art" equipment is always going to be difficult to cost; things evolve as development progresses.