My daughter has joined the Southwark Youth Orchestra (not to be confused with the South London Youth Orchestra). It's free, which is great and meets in summer hols/half terms. It's quite new, only got going this year, has a grade 4/5 entry level and is relatively low key, compared to say the Dulwich Youth Orchestra, which she has also tried and enjoyed but found more demanding. For info on the Southwark Youth Orchestra email:
[email protected] or I think the Dulwich Youth Orchestra is also good - don't have the email to hand, but they have a website if you google them. They also meet in the summer hols but they are not free though. Good Luck!