Computer Misuse Act (CMA), established in 1990, includes:
Unauthorised access to computer material- This is basically hacking. It is illegal when it is done without authorisation.
Unauthorised access with intent to commit or facilitate the commission of further offenses- If you hack someone with malicious purposes, even with their permission it is against the law.
Unauthorised acts with intent to impair or with recklessness as to impairing the operation of a computer- Any unauthorised changes like sending a virus.