I dont have the data, but Oxford Brookes is actually an exception and I don't think the only one. It's awash with money because of the sheer volume of domestic wealthy attendees with various donations to various causes. Sheffield Hallam and most of the other polys aren't usually in quite the same position and your point stands.
I've looked at the data for some of the Russell Group and, coming from a US perspective, I was rather shocked at how reliant even top UK universities are on tuition. Apart from Oxbridge, they mostly don't have anywhere near the cashflow from endowments or alumni donations as the US Ivy League does.
