It's a bit of a grey area, legally speaking. I was always under the impression that it's only an offence if someone complained.
Wikipedia says;
"In the United Kingdom, public sex comes under laws related to
voyeurism,
exhibitionism or public displays of sexual behaviour, but public sex laws are ambiguous. Prosecution is possible for a number of offences under section 5 of the
Public Order Act 1986,
exposure under section 66 of the Sexual Offences Act 2003, or under the common law offence of
outraging public decency. The policy of the Association of Chief Police Officers (
ACPO) is that arrests are a last resort and a more gradual approach should be taken in such circumstances."
(Please bear in mind, folks, that Wikipedia can be unreliable)