Funding in mathematics from large companies as undergraduate

Charitable funding from large companies seems unlikely

In the US, only about 4% of scholarship dollars come from private sources.

Here is the breakdown of sources of college grants: federal government (44 percent), colleges (36 percent), state governments (9 percent), employers (7 percent), and private scholarships (4 percent).

-"7 Things You Need to Know About Private Scholarships", U.S. News (2011-02-01)

So, you're in the UK and asking about summer research funding, but it seems likely that the same trend would apply: that not much of the funding would come from corporations.

The big problem's that there doesn't seem to be a reason why they'd do it. Even if a company wants to give its money away to research, it seems like they'd target medical research or something else with near-term applications.

You may want to think out an argument for your research

In general, the problem's that capital's a finite resource; people could just as easily put their economic power toward medical research or their own interests. So, why should anyone want to give you money?

This isn't rhetorical or to make you feel bad! Rather, it's an important life question that we all have to ask. And if you can find a good answer to it - one that isn't biased by your own personal wants, but rather is objectively compelling - then that'll give you a basis for pursuing sponsors.