What counts as unauthorised access to a computer in the UK?

Causing a computer to perform any function with intent to secure access to a program or data, knowing that the access is unauthorised, is an offence even where nothing is taken. The offence is aggravated where the access is intended to commit or facilitate a further offence, and a separate offence covers unauthorised acts impairing the operation of a computer. Penetration testing without written authority from the system owner falls inside the offence.