Just adding my 2 cents. Since you're in England, find a European city or similar where you'd like to go. We are currently travelling here and have met a few English people who say they were able to get return tickets to Krakow in Poland, 4 days accommodation and a day tour for a grand total of 200 pounds. Another mode of transport to look at is megabus. They do cheap travel from many places in England. So I'm sure you will find something.
If you really really want to do Mexico as your first trip, go for it, and I highly recommend going solo or with a friend or 2, but don't do any organised group tours. They are expensive and realistically most of the stuff you can do on your own if you are willing to spend time researching online. If you stay at backpackers or hostels, you will find similar minded people. We travelled earlier in 2014 for 4 months in South and Central America. We had little knowledge of Spanish and just picked up on the way...it also helps to have offline google translate as I found myself stuck many times and I just typed it in English and got them to read the Spanish translation. Sign language is universal.
To end, you are going to get a lot of advice of where to go and what to do. But, I'd suggest first see where you want to go, how long can you afford to go for and how much money are you willing to spend (have a 10% reserve, for all those cool things you will want to do when you find them)
I find trip advisor to be helpful for activities.