This attitude is what is wrong with Britain today, many young people feel they are too good for a job, when in reality they haven't proven themselves yet and therefor are not too good for ANY job.
So many graduates whining that they can't get a career in their field, but have never had ANY job what so ever, sure volunteering and charity work and societies are all good. But what a potential employer wants to see, is an employment, something that's going to present an image of your work ethic. If you have done volunteer work and such, its good, but most employers know that the company or organisation will feel obliged to give you a good reference because you worked for free.
However if you have 2 or 3 years in some minimum wage job, that you probably didn't enjoy, but still have a good reference, that immediatly tells the employer, okay, this person is capable of knuckling down, getting on with work and performing when things are ****.
I've worked at McDonald's, it's not the worst job in the world, I have also worked in factorys, cleaning warhouses, cleaning offices, driving lorries, security and call centres. And some of those jobs were much worse than McDonald's, I have peers who are stuggling on JSA because they get rejected from every job they apply to, I fire off a couple of Emails and I have enough job interviews that I have had to turn some down.
Even if you think a job is not relavent to your ambitions, work experience has so many secondary benefits when jobseeking in the future and employers of any level appreciate evidence of hard work.
Good luck with the job, McDonalds is actually an excellent job for students due to its flexible shifts, if you let your managment know in time, you should be able to easily get exam times off work.