[kbbgl87]

> a game where you had to fire a cannon (choose angle and power) and hit something.

Scortched Earth?

[phatskat]

Scorched Earth was such a fun game! Many hours were lost at our small home LAN parties playing SE and Quake/2/3

[rkomorn]

Or one of its predecessors, Tanx.

[adzm]

Based on the age of the bbc micro, no way it was scorched earth, tanx seems likely (I think it was 3d tanx? I have a vague memory of seeing this in a vintage collection)