Many people have said that this word means 'leave it out' or 'don't bother'. We it does mean this but it is also used as a sign of agreement to a situation.
It largely depends on the tone
So it can either mean 'fuck that', 'can't be arsed' or on the other had 'yeah mate', 'lets have some of that'
If said in the negative term it would be said with a lowering in the pitch of voice towards the end of the phrase, if however said in the positive it would be said with a rise in the pitch of the voice towards the end of the phrase.
As well as this in the positive it is accompanied by the whip thing chavs do on the 'that'
Jim: Mate have u got dat new playstation game where ya like kill people
Ben: Yeah mate allow that (pulls off a whip)
Benson: You know we gotta be up at 6am tomorrow!!!
Hedges: Allow that
A kind of underground culture developing or developed around east london. Can be linked with Indie and has a large musical community surrounding it. These include The Libertines, Babyshambles, The Rakes adn other such bands with hard hitting simple drums and scratchy messy guitar playing. Lyrics are usually centred around going out on the town or about city life in general but can also be deeper. The word may originate from the word urchin because if u look at people such as the libertines they could be described in some way as an urchin of the street.
The Rakes album Capture/Release sounds like a Metropolitan Tube Train travelling at full speed through the depths of the underground with the dead carcass of the urch scene underneath its rusted wheels.