Yes - Exposure is an offence under the Sexual Offences Act. To be honest, I don't think the Public Order Act really covers willies out.
There are three main offences (before we get into the realms of affray/riot, where the threat of violence is required).
Section 5 - "using threatening, abusive or insulting words or behaviour in the hearing/sight of a person likely to be caused harassment, alarm or distress."
Nakedness, unless it is accompanied by other unpleasant behaviour isn't "threatening, abusive or insulting".
If it becomes any of these, things - i.e. a man outside a school showing school children his penis - then it is already covered under Exposure. Plus, section 5 can only be dealt with by way of fine.
Next offence is s4A, which is "intentionally" causing harassment, alarm or distress - again, if you intend to upset someone, you are guilty under exposure.
Last offence is s4, for which there must be a fear or threat of violence.
So in essence, the Public Order Act doesn't go any further than the Exposure legislation. The prosecution would still have to prove beyond doubt that the defendant intended to cause alarm or distress.