I've been looking around for flights from the UK to Mexico in spring next year (either Mexico City or Cancun), and I am finding the cheapest to be over £650 whether scheduled or charter. Does anyone know, is this the going rate these days, or am I missing a good trick somewhere?! I am looking to travel in the South (Yucatan) rather than near the US border.

Any advice or suggestions much appreciated!

We went to Mexico this summer and we've payed 860€ per person. Quite a lot, if you know that the ticket had gone up from 730€ to 860€ in one week. We've booked our tickets in December to leave in July.
It is cheaper if you arrive and leave at the same airport. But if you want to travel in the south (Yucatan), you'd better fly to Cancun, as Mexico City is quite far to start from and to go back if you travel in Yucatan.
Book the tickets as early as possible. But if you want to fly outside the high season, you might wait to see if you can catch a last minute. But we were told that Cancun is a very touristic airport and that last minute tickets are quite rare.
Fortunately, we hadn't waited to buy our tickets, because they went up to more than a 1000€ for the same flight as we had booked.