It's not clear if spoofing is illegal? It seems as clear as crystal to me.
My telephone is a computer, as are most phones these days. One of its jobs is to make a record of the number of all callers so that I can return calls. Spoofing deliberately causes a false number to be recorded. I have never authorised anyone to record false data in my computer's record of callers.
S3 of the Computer Misuse Act 1990. Unauthorised acts with intent to impair, or with recklessness as to impairing, operation of computer, etc.
(1)A person is guilty of an offence if—
(a)he does any unauthorised act in relation to a computer;
(b)at the time when he does the act he knows that it is unauthorised; and
(c)either subsection (2) or subsection (3) below applies.
(2)This subsection applies if the person intends by doing the act—
(c)to impair the operation of any such program or the reliability of any such data
It could not be any clearer, spoofing is an offence under the Computer Misuse Act 1990.