The existing Act of Parliament that could be used to penalise revenge porn is the Communications Act 2003, Section 127. This states that:
A person is guilty of an offence if he—
(a) sends by means of a public electronic communications network a message or other matter that is grossly offensive or of an indecent, obscene or menacing character; or
(b) causes any such message or matter to be so sent.
This legislation has been used successfully against several thousand people who have posted, tweeted etc messages that have been deemed to be "grossly offensive" or "menacing".
Using this legislation to prosecute sending of "indecent" messages or other matter (images), while theoretically possible, would criminalise vitually all internet porn, adult chat-lines etc. This is probably not what Parliament intended, so it would be left to a Judge to sort out the mess. So far, some Judges have shown some reasonable common sense in this - as the Lord Cheif Justice did in the famous "Robin Hood Airport b***" joke tweet, although some judges have made a dog's dinner of it.
A further problem is that the offence relates to the use of a "public electronic communications network" the definition of which is a complete mess. (E.g. Twitter itself is not a PUBLIC electronic communications network, but sending a grossly offensive tweet via, say, Talk-Talk, would be an offence, because, like most ISPs, Talk-Talk is a PUBLIC electronic communications network.)