Most won't due to liability issues. Has your cylinder been tested and recertified? Can you prove it?
They don't want to fill a cylinder with high pressure gas which they have no idea of the history. They could seriously injure themselves or be held responsible for injuring someone else.
Does BOC let you buy a bottle with a deposit or must it be on a monthly rental scheme? For the small quantity I use it would be cheaper buying disposable welding gas bottles.