My memory is that the TriStar was purchased in the post-Falklands euphoria to satisfy a particular specification which was to tank a Herc to Port Stanley, fail to get in, then tank it back.
Very shortly after they bought the TriStars they started building MPA. The word on the street at the time was that no-one wanted to go back and change the spec to a three point air defence/strategic tanker in case their Airships noticed the original specification no longer applied. No RAF Port Stanley, no need for TriStar tankers.