Hi.
I've just dropped out of uni after trying to do maths and music at Keele. My parents were not overly impressed, especially my dad. My family do give me some stick about dropping out, but like you seem to be, I intend to return to uni next year to do just music (not at Keele though).
I would say don't drop out until you're sure. Maybe speak to someone at your uni and family/friends to look at your options. Worst case scenario, you stay the year, and then transfer to another uni for next year.
If you did choose to do music, whilst it is difficult to get a career in performance, it is doable, and obviously does happen. There's also lots of other things you can do though, especially if you find you really enjoy composition, there's lots you can do with that. There's plenty of jobs you can get with a degree in music, if that's the route you choose to take. I'm afraid I don't know much about archaeology, but I imagine its a similar story (eg. working in museums etc).
As long as you're at least grade 7-8 (depending where you go) and have at least grade 5 theory/knowledge by the time you start uni, many won't mind you don't have a-level music.