Thanks! Based on the additional information I have tracked it down

Author : Nicholas Fisk
Title: Space Hostages

A crazed and dying Flight Lieutenant, nine village children, a top-secret spacecraft - all of them out of control and adrift in space! Someone must take charge. But who?

Brylo has the brains, but not the personality, so it is the powerful young bully Tony who sets himself up as Captain - and steers the ship and its cargo of children towards new and horrifying dangers...

Published in 1967