Amin was, of course, welcomed by the British as a better president than his predecessor, Milton Obote. It was "a good thing" to have a Sandhurst trained army officer in charge.
That's not to say that Obote wasn't an evil schemer. He murdered his predecessor (who was exiled in London at the time) - in the flat above my grandmother, in Rotherhithe, as it happens. When Obote's people turned up to do the deed, they actually got the wrong flat and the old dear had to redirect them upstairs!
You're right about the British Govermment's support of Armin. It's akin to the US being big buddies with Saddam when said tyrant was anti Ayotollah. Hypercritical isn't it ?