Income tax rules when spouses are tax residents for different countries (specifically France and UK)

I know tax laws are country specific and in case of spouses living in two separate countries they may depend on the specific countries due to tax treaties so my question cannot be asked in all generality so I will make it specific to France and UK.

If I go to work in UK and my wife stays in France with our child how will it effect computation of taxes for both of us. I know in France for instance the taxes are computed using household units and in our case we count for 2.5 units. I know taxes in France will be due only on the French part of the income but will it be computed using 2.5 units or 1.5?

If the latter then the tax burden becomes much more for spouses that have different tax residency.