Xanda
Just to expand on the 767 issue a little.
767 can make Cancun without undue difficulty especially with winglets and reduced capacity in the cabin to make way for premium class. On a bad day however, wet, low pressure, 90 degree crosswind ( standard BHX ! ) then things can get a little tight depending upon the fuel requirement for a given sector on a given day. FCA used to push their longhaul up to EMA which had a longer runway in the correct direction ( prevailing winds) - the much trumpeted runway extension will make life easier if it ever materialises and maybe just maybe the economic situation in the Midlands will have improved and load factors will rise.