It is not illegl to have sex in a public place.
The Sexual Offences Act (2003) doesn't legislate specifically against this practice. Public order offences exist that guard against outraging taste and decency (i.e. shagging in the supermarket aisle), but permits sex in an isolated place so long as you have a reasonable expectation of privacy.
Having sex in a car or having sex in a public place is not in itself illegal. To allow a willing person to watch that act can not be illegal. What is illegal is if that act be seen by a third party who is able to report the matter as something that has caused them offence or intimidation. To expose yourself to a willing person is not illegal, but it seen by a third party it could be construed as indecent exposure.