It really does depend on a number of variables such as : Winds aloft; traffic flow; sig weather avoidance; etc.
The more common routing would be out towards lands end then across the Atlantic towards the East of Bermuda, On towards Nassau Bahamas, then across the Florida keys to a point just North of Havana and North West Cuba. Then straight on down towards the Yucatan Peninsula and Cancun itself.
There are some strong South Westerly winds in a South West to North East flow pattern (Jetstream) on this route iin the morning from the UK out to about 45 degrees West, and it may prove more beneficial to follow the North Atlantic organised track system to Canada and follow the eastern seaboard down to Florida then on towards Cuba and Cancun ? Possibly a combination of the two ?
It really is very difficult to say at this point in time.
Flight time should be somewhere in the order of 9hr 45 mins give or take a little.
Anyway enjoy your holiday.