Mexico City has a fairly decent tech startup scene, with plenty of jobs available. There's also a good amount of jobs in a more 'traditional' sectors as well as IT consulting firms.
Depending on your knowledge of Spanish you'll want to apply to a small firm or a big corp. Big corps would be slightly more likely to be easier for a not-so-proficient Spanish speaker (but keep your expectations low), smaller shops to a lot lesser degree. Most tech startups would probably be a better fit for you if you are not very proficient and could also be more flexible, open to work with foreigners. Not surprisingly, tech startups tend to be more in line with a more modern, globalised way of working, so you'll be more likely to feel at ease and face way less language/cultural barriers.
Cost of living is lower than many other major cities, and salaries are accordingly lower to what you'll be making in other parts of the world. This has two implications:
Since some (not all) goods and services are either directly priced in dollars or affected by the exchange rate, whenever the Mexican peso (MXN) devaluates with respect to US dollars (USD), your life expenses will increase, (recently, we had a ~15% devaluation).
Traveling (or relocating) to other parts of the world will be more difficult, since the money you earn here won't get you as far as you GBPs get you know.
Google USD to MXN to see the current exchange rate and the trend.
Side note: Apparently, if you are an Advance customer, HSBC can help you to open accounts and transfer your credit history wherever they have a branch. I think American Express can do a similar thing, but obviously do check with them. Disclaimer: I have no affiliation to HSBC nor Amex.
Welcoming as Mexico and it's people are, as is the case in many countries you'll need a work permit to legally work here. You'll need to contact National Immigration Institute (Instituto Nacional de Migración INM).
There's the possibility of going the informal way, but bear in mind that you won't have access to some services as banking, etc, and you'll always need someone to sign contracts and contract services on your behalf. In my book, it's not worth the hassle.
I'm a local so I don't have hands on experience about applying to work/live here as an expat, save for what some foreign friends have told me. That being said, I think the information on this link http://www.transitionsabroad.com/listings/work/articles/working-legally-in-mexico.shtml would be very helpful in addition to what others might add here.
All that being said, despite the inconveniences, Mexico City is a vibrant, colourful and friendly place to live.
I wish you the best of luck.