Just go for a masters and take what you want from the undergrad list.
An in-between approach would be to spend a year or two working on the things you'd like to brush up then apply for postgrad. If you feel that you didn't get good instrumental teaching, have private lessons with a teacher who you know is good. If it's another aspect, then enrol for some courses at an adult education college such as City Lit or Morley College, or a distance learning one if you don't have somewhere nearby. It'll be cheaper than doing another undergrad at full overseas fees.