Funding
Universities themselves may or may not have money available to employ students over the summer, but there are a few organisations who have bursary schemes for just this purpose.
The Nuffield Foundation offers Undergraduate Science Bursaries to students who are thinking about a research career.
See their website for what they offer, how to apply and some experiences of students who have received bursaries.
There is also a list of some other funders offering vacation bursaries on the Nuffield Foundation website.
The Royal Society also has Summer Studentships available, through their University Research Fellows. In common with the Nuffield bursary scheme, you cannot apply for an award alone - the supervisor for your summer project would have to make the application. So these are not advertised on their website.
So - first find a supervisor! This might take a bit more pre-planning than signing on with a temp agency for the summer, but it could help you make decisions about your future and will look good on your CV, when you apply for postgraduate places or for other science-related jobs.