Once you have a Live-In job in the UK it is quite possible you may want to look to for work as a Personal Trainer on the side. Several of my friends were able to get Live-In jobs in small villages pubs from where they were able to start little P.T. businesses. Every small village has a ruby or soccer team and they are all based out of the local pub, in fact everything in small villages is based out of the local pub. Often these local teams lack for coaching, especially in areas like sprint training, and so there are opportunities for P.T.s also it is a great way to meet people and get involved in village life.
I do know one or two people who got work in upmarket Hotels on the Channel Coast as Personal Trainers and Pool Attendants. Believe it or not being Australian is a real advantage in the U.K. Hotel market, people see Australian P.T.s as offering something different from local trainers. Although working for a Resort Hotel on the Channel Coast must be regarded as a top gig, I think the reality is that they are a little difficult to come by.
There are several web-sites and companies that help people get Live-In jobs, a few of the better known ones are Workabout, or Global Work and Travel. Please do see this as a recommendation I am merely saying it is quite possible to find work on the Web before you leave for your holiday. Now it might cost a small amount of money to register on a couple of these websites, but the word is small, and you should always be paid directly not through the travel company. Even though you might be considered a back packer you can still use the Department of Work and Pensions to you feel you are being underpaid or are somehow being taken advantage of.
As for working in the larger cities like London I cannot be of much help. Finally you will need a working visa which you must apply for before you leave.