Xanda Man. Hope this helps-
BHX runway is shorter than MAN. The longer the runway the more weight and distance can be flown. ( BHX runway extension begins next year so this will not be an issue soon )
I know of 3 seperate reasons why 'sometimes' the CUN flight goes via MAN-
1 - Crew utilisation. Depending on crewing patterns the flight sometimes is crewed by MAN crew and will be tied in with 2 and 3 below.
2 - Weight. This flight is on a such fine margin with weight verses BHXs runway length. If the flight is roughly 95% full OR LESS, little or no cargo, and weather predictions on route are pretty standard then it can operate non stop to CUN with or without winglets. However with winglets does offer a 5% advantage in regards to fuel burn.
3 - Cargo. If a significant amount of cargo is booked for this flight it is loaded at MAN. As MAN runway is longer than BHX this flight can operate fully laden no problem.
As a pax on this flight, going non stop is perfect as once I went via MAN and I was onbourd for 14.5hrs! When in MAN pax are kept on board whilst cargo and fuel are loaded and crew changed. This was a very long day and too long for young families IMHO. Return flight to BHX was a breeze at 9.5hrs.
Like I said, when BHX runway extension is completed these issues will be solved for good. About time too!