Seriously, forget the calculation, forget the BUPA check (unless that established your MHR when cycling). Strap on your monitor and go for a ride (on the road1), get warmed up, do some hard efforts to get your heart rate up when heading for a long steepish hill. Then hit the hill as hard as you can, don't worry about getting to the top, sprint as hard as you can, when you think you can't go on put in one last effort 2 until you really can't go on. When you can see straight again, check the max rate and that'll be fairly close providing the hill was long enough and you really did try hard enough. Let us know how the result compared to theoretical calculations. Mine was 205 when I was 47. Means nothing, it's an individual thing, but at least you'll know what your "zones" really are.
As amaferanga says, if you have limited time you'll burn more fat with hard effort. If you have all day, you'll eventually burn more fat riding gently because you'll be able to keep going a lot longer.
1 On a turbo I can only get within 2 or 3 of my max measured on a proper ride, not sure why
2 Assumes you have no known or unknown health problems, at your own risk