London is as expensive as you make it. As I’m sure will be the case with Magic in Brisbane, the earlier you book, the cheaper you’ll get it. People moaning about London and staying in London baffle me, in all honesty. You’re in a very well connected city and transport is relatively easy (if a little scary looking if it’s new to you). For example, two nights in a Premier Inn (chain hotel) in Wembley for the Challenge Cup Final is more than double the cost of a Travelodge (Premier Inn’s competition) in Stratford. From Stratford, you can get the Jubilee Line direct to Wembley Park for about £8-9 on a Saturday. As will be the case everywhere, you make these things as cheap or as expensive as your budget can take. A Wetherspoons meal and a pint of beer will set you back £10-20, a McDonald’s £8-14 and a restaurant around Oxford Circus £30+ per person.
Trains are expensive to and from the north but again, if you’re organised and look into railcards, you can get some decent deals. Alternatively, drive somewhere south, find a secure car park for a couple of nights and do the last hour of the journey into London by train/tube.
There’s a real attitude problem with people in RL from the north. Cup finals and internationals are announced in advance. Sometimes way in advance. There’s plenty of time to work out routes, save money and organise proper trips.