You would need around €1,368.43 (£1,194.92) in United Kingdom to maintain the same standard of life that you can have with €1,341.60 in France (assuming you rent a place to live in both cities). This assumes net earnings (after income tax).
If you eat out 4x a week in an inexpensive, and 1x a week in a mid-range restaurant you will pay €11.09 more in United Kingdom (€83.59) than in France (€72.50).
For monthly supply of groceries worth €223.60 in France you will pay €40.24 less in United Kingdom (€183.36). If you are an average smoker (1 pack per day), you'll spend €99.20 more on cigarettes each month than in France.
While using public transportation in United Kingdom using the monthly pass you'll end up paying €6.81 more than in France. However, if you will be driving a car 32 km on average each work-day, each month you'll spend €0.10 less on gasoline than in France.
To buy an 85m2 apartment in United Kingdom city centre you will need €354,214.40 which is €59,724.55 less than in France (€413,938.95), however to buy one outside of the city centre you will need €261,437.87 which is €2,289.87 more than in France (€259,148.00).