If they (Virgin) claim the issue is not down to them, strongly reiterate that nothing in your set up has altered other than your (current
) choice of ISP. Ask them to follow the logic of your statement step by step, and explain to you where they disagree.
Sometimes firstline support at an ISP either don't know about their company's bandwidth shaping policies, or are under instructions to deny they exist. Unless you signed up to a long term contract with them, if they don't want to assist you and carry out their own problem tracing, remind them you are free to shop around to other internet suppliers
(I've worked at an ISP previously).