We got our tickets in July for the December gig. For sold out tickets you can often get them on ents24 for a huge fee.
I noticed that they are selling Example tickets in Manchester for £67.
The other alternative is touts outside the show.
Some of the gigs have an age restriction and it isn't necessary down to the artist. eg to see Professor Green at 53 degrees in Preston you have to be over 16 and have id. The night before (I think it is) he is in Manchester and you have to be over 14.
Back to example. I love him and find his lyrics witty but he can be explicit. I probably would have said no to an 11 year old. My son is 15 and he is going.