You can get a gun easily from an arms dealer or from a fellow gang member. Literally costs less £300 in some cases.
There are 2 main sources of guns in UK:
1. Antique Guns
There are a lot of WW2 guns in circulation on london streets. WW2 veterans were allowed to keep their sevice rifles as antiques. You are allowed to own and trade antique guns in the UK, no problem. These then find their way on to the streets.
2. Eastern Europe
Cadaan gansgters from Essex bring guns from legal markets in eastern Europe into London by boat. These guns are more modern in comparison.