What does 'Tatts Out' mean?

Many Londoners have puzzled over this poster in recent weeks:

So I tested it on my long-standing, but often sitting adverts focus group. Here's the transcript:

 Leader: what do you make of this advert?
- The sun's not out, they've got that wrong.
- They must've thought August would be sunny. 
- If they thought it would be sunny, why is he getting rained on?
- Is that rain?
- He might be in a shower.
- That's a nasty burn he's got on his chest. Maybe the shower is helping.

 Leader (sigh): let's move on.
- He looks like he wants a fight.  I often feel like punching, in the shower.
- Is he on the phone, though?  
- Don't be stupid, that's a fist, ready to hit someone.
- Is he that boxer?
- Is it about gang violence?
- That's how he got the burn, isn't it.  No wonder he's angry.

 Leader (prompting): What do you think 'Tatts' means?
- No idea.
- Tattinger. He's just won a fight, if he's that boxer. 
- Taters - potatoes, I mean.  That's what we call them in Yorkshire.
- Is it a mistake - did they mean twats?

Leader (forcibly): Can you see the L'Oreal products?
- L'Oreal?  That's for women, isn't it?  Creams and all that.
- Must be to help his burn.
- Is it saying if you feel angry about your burns, don't go punching anyone, jump in the shower and put some cream on?

The group is hurridly closed.