As has been mentioned, the offence of “Exposure” is covered by section 66 of the Sexual Offences Act 2003. The section is very brief and succinct. It states:
A person commits an offence if—
(a) he intentionally exposes his genitals, and
(b) he intends that someone will see them and be caused alarm or distress.
There is no need for the offence to be carried out in a public place. For a successful prosecution it will have to be proved that he exposed himself intentionally and that he intended someone to see him and be caused alarm or distress.
From your description I think the main issue to be overcome will be whether or not he did what you say. It will be that of his intention to cause alarm or distress. If, as you say, his actions are directed towards the Polish girls it may be very difficult to prove that he intended them to be caused alarm or distress. Although it may be clear that other people are being caused alarm or distress it might be tricky to prove that he intended them to be so affected.
Your local police may be able to cure this problem by paying him a visit