Peeing in public has been an offence for a little while hasn't it? Economist even had an article on it this week, albeit more from the angle of being outside whilst the public toilets are generally shut, and the resultant increase in people going wherever, but still.
A nation is caught short
www.economist.com
Yes. In most cases, it's a sect 5. Public Order with disorderly conduct (causing harassment, alarm of distress): summary offence so fixed penalty notice or a bench.
Drunk and Disorderly is another option, but it is a found committing offence (prevent it from continuing), so you have to arrest at the time - can't do it afterwards.
Let's be honest, he probably didn't know it was there so it will depends if he's summonsed or charged to the crown: the former will probably go down to Sect 5.
Like I mentioned earlier though, outraging public decency is an indictable under common law so unlimited fine and/or maximum of life imprisonment.
If he's found guilty by a jury, which he would as the evidence is there, a heavy fine is on the cards or if he has previous maybe... just maybe... short custodial.