I used to work as an IT recruitment consultant in London, though that was about 10 years ago, but I'll try to give you some advice.
First, Math degree from Warwick, with a 2:1 or 1st. With a grade like this you could well be looking at working in a good city financial firm, the Square Mile or Canary Wharf. Many of them stipulate that they want at least 2:1 degrees, even after years of experience. With a Math background, I would also suspect these kinds of institutions would be looking at you for things like C and Java, rather than .NET. Most of this work would probably be on UNIX, and for finance houses this usually means Solaris.
As for Mac development, in the UK these opportunities are few and far between, though I have noticed a marked increase since I left the industry. Most of the Mac work is at operator level. Development work tends to be cross platform, web based applications, and they usually have a phenomenally long list of technologies involved.
To begin with you should look for a permanent position, not contract. This will give you commercial experience. This is what employers want. They want someone who has done a very similar project before, knows all the problems and how to overcome them. Permanent positions will also allow you exposure to a much broader range of technologies, and usually better projects, not just bug fixing.
As for travel, if I were in your position, I would take a job first, for a year or 18 months. Get some commercial experience, a bit of money in the bank, and then travel. I don't know where you want to go, but (and I am assuming you are around 21 years old) with some experience behind you, you could do something like go to Australia on a 1 year work visa and easily get a job there doing more or less the same thing. I worked with many developers who had done this, and the UK is full of Aussies, Kiwis, and S. Africans who are doing the same, just in the opposite direction. In fact, there is an organisation in London where they go to register the moment they step foot off the plane, which then forwards their details to all the agents in the UK. I am certain there is an equivalent agent in Australia or New Zealand. With the benefit of working in a good job in your travel time you could extend your travel significantly, and when you get back to the UK have more experience under your belt.
Look at
http://www.jobserve.com. This is a site all the agents used to use. There are many others that have sprung up in the 10 years I've been away from this game, but this is still used by just about everyone. It will give you a good idea of what kinds of jobs are out there. Don't get carried away by the salaries they are advertising!
With your degree, and if you do get a 2:1 or 1st, I really don't think you will have a great deal of difficulty entering the IT market. Mac development in particular may be a different story, but with something like C or Java, you will be OK.
I hope this helps.