[bell-cot]

My recollection is that decision was a straight-up "success is irrelevant, HM Treasury has decided that the UK cannot afford to have an orbital launch program".

Wikipedia's version: https://en.wikipedia.org/wiki/Black_Arrow#Cancellation

From the timing, one certainly could speculate that the needed pounds were being squandered on other programs - ones given higher prior, but less-competent leadership.