To apply for most conservatoires, you need to apply though UCAS Conservatoires. I'm fairly sure you won't be able to do this whilst holding a confirmed place. Guildhall has a separate process, so you could apply there, however the deadline for music courses at conservatoires has passed, it's before oxbridge/med students even put theirs in.
If you did choose to apply, you would need to contact the conservatoires and ask if they have any spaces/auditions left for your instrument. It's also worth checking whether you can apply through conservatoires with a place in normal ucas.
What do you play?