You would need around €1,277.61 (£1,123.74) in United Kingdom to maintain the same standard of life that you can have with €1,264.97 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 €10.49 more in United Kingdom (€82.99) than in France (€72.50).
For monthly supply of groceries worth €210.82 in France you will pay €37.94 less in United Kingdom (€172.88). If you are an average smoker (1 pack per day), you'll spend €96.96 more on cigarettes each month than in France.
While using public transportation in United Kingdom using the monthly pass you'll end up paying €6.31 more than in France. However, if you will be driving a car 32 km on average each work-day, each month you'll spend €0.56 less on gasoline than in France.
To buy an 85m2 apartment in United Kingdom city centre you will need €351,653.44 which is €62,285.51 less than in France (€413,938.95), however to buy one outside of the city centre you will need €259,547.68 which is €399.68 more than in France (€259,148.00).