University Summer Schools

I would be interested in attending a University Summer School for Sciences. My question is:
How worthwhile are they ie are they more beneficial than a work experience placements at a GP or a Dental Clinic?

That depends on what exactly you want to achieve at a uni summer school. In my own opinion I'd say they are very very useful, as they will give you a lot of resources. I went to the Nottingham summer school to learn more about medical science for a week, had the opportunity to prepare for UKCAT and BMAT as well as receive very useful tips (although in the end I didn't choose to do anything medical related and chose Maths ). The best thing was that we got the chance to see an actual dead body and touch it, as well as carry out some simple tests they do in the hospital (those interested in dentistry did a similar thing). Other than that, we were assigned a mentor for a whole year after the summer school had finished, were entitled for a bursary (only available to those that did the summer school), and some even had their offers reduced.

Other than that, it all depends on which summer school you go to and what you want to gain from it. My own advice to you will be to not neglect work experience (you are still required to do it, regardless whether you are doing a summer school or not).