Uridium

Uridium is a video game developed by Andrew Braybrook and ported to several home computers. It was converted for the BBC Microcomputer by AIM and published by Hewson.

Gameplay:

Your home solar system of 15 planets has been attacked by a horde of Super Dreadnoughts, from a race who wish to harvest your universe's minerals for their own use.

You must pilot your craft through some of the hardest levels ever created, shoot down enemy spacecraft and ground weapons, and avoid the many indestructible hazards. When a sufficient number of enemies have been defeated, the player can then land on the Dreadnought and proceed to destroy the ship's reactor.

The innovation is that you do not simply travel in a single direction, but instead move from left to right or right to left depending on where the current targets are, in a manner more similar to Defender than most shoot 'em ups.