"allow it" means to just leave something alone, or don't worry about it. People in West London say this quite often.

widely used by working class youth amongst the london area. contrary to the standard definition of 'allow', the slang phrase 'allow it' is often exclaimed to express unwillingness, dislike or general negative opinions towards any given subject.

can also be exchanged for 'allow that', however this is generally considered an ill conceived middle class variant.

It basically means lyk to leave summink coz it's out of order or it's a long ting.

something whiny school kids say when they want something they're not going to ever get

Forget somthing or leave it alone