I would say a), and use any money left over for a nice trip to Spain in the summer before you go to uni to learn about the culture of it all. If you want to put something on your personal statement, a teach yourself course is probably just as valuable as going to lessons, and it shows that you have the initiative and commitment to pick it up yourself. Plus, you can do it at your own pace. If you’re worried about not knowing any Spanish before you go to uni, don’t be, there will probably be people of all levels in an ab-initio class, some with a GCSE, some who have had a few classes and some who know nothing at all, so you will learn everything very intensively and very quickly once you start uni. Basically, I think your money and time is better spent elsewhere. I assume you are doing a-levels right now? Its better to concentrate on those rather than get an unnecessary qualification. I did ab initio Spanish, and I did go to evening classes beforehand but I had a gap year so I had a lot of time on my hands. My teacher was very good and I learnt a lot which gave me a bit of a head start once I started my degree, but there were no exams and I didn’t get a gcse or anything, just a Open College Network certificate. If you stick at it you can learn a lot from a book. I used one called Pasos which by coincidence ended up being the course book for my evening classes, and it was good for learning the basics. And do the usual watch films, listen to music blah blah to pick it up.
Maybe if you really want to do a course you can think about these one or two week courses you can do in the country next summer? You wouldn’t get a qualification but it would probably be a lot more fun.
The most important thing is once you start your course at uni keep on top of it, you do learn a hell of a lot and in my course we were with post a level students in the second year, so regardless of there knowledge of Spanish before the start of uni, the students who didn’t work hard in the language classes fell behind very quickly, and those who kept track of what they were doing fared far better.