On the same 65 000 € gross, a worker takes home roughly £10,001 more per year in United Kingdom than in France. That gap reflects the tax structure alone — before rent, healthcare, or savings behaviour come into play.
Housing is the first multiplier. France has high rent pressure, while United Kingdom has high rent pressure. The two markets behave similarly, so most of the gross-to-net advantage flows straight into disposable income.
Healthcare and pensions go in the opposite direction. France runs a universal healthcare model — Universal healthcare with strong public reimbursement; supplementary mutuelle is common. United Kingdom uses a universal model — NHS provides universal care funded from general taxation, with private top-ups. The country with lower take-home often shifts costs that the other country leaves to your private budget.
Net-of-everything, a relocation decision should weigh comprehensive welfare state in France against strong public welfare in United Kingdom, plus differences in pension capture, social safety nets, and city-level cost of living.