Hi RMC,
I'm assuming that you earn a gross salary in the UK of GBP 80K and that you pay tax at the rate of 40%, leaving you with GBP 48K net. This equates to GBP 240K over 5 years. From this GBP 240K, you would have to subtract your normal UK living costs to find out what you would save in the UK over 5 years.
If you only spent half of what you earn, you would be left with GBP 120K.
At FlyDubai you would earn AED 660K per annum net of tax. This equates to AED 3,300,000 over 5 years.(GBP 550K). You have assumed a spend of AED 26,800 per month on living costs. At the end of 5 years you would save AED 1,608,000(GBP 268K).
Your house in the UK and the mortgage are irrelevant because you will have these costs regardless of where you live (assuming you don't sell it). You should really factor in the UK rent as additional Dubai income, because you would not collect this rent if you were living in the UK.
By my calculations, you are more than GBP 148K better off after 5 years in the desert. That is assuming all goes well with FlyDubai.