Sorry to be pandantic, but manda37 is a little wrong. there are 5 laws I can think of that will cover it
1) Under Sexual Offences Act, Yes, somebody has to be offended but nobody has to be there to be offended, if that makes sense.
For example, a police officer could arrest you on the supposition that somebody could be offended and it is best practice for an "offendee" to be present, but not legally necessary.
2) Other laws that could be used, Public Order. However, sect 5 (displaying anything that will cause alarm, harassment or distress), you should be warned first to cover up. Then no offence.
sect 4 (public order) no warning is necessary but you will have to "intend" to cause harassment alarm or distress.
Common law "Public Nuisnace", being a pain in the ar5e basically
and "Furious Driving", driving in a manner likely ti infuriate other road users.
The law may have changed since I read it, but as the greatest criminal mind on AB, I will leave it to you.