Is one first class stamp enough to send a normal sized letter to france?
No. As of 4 April 2012, provided the letter does not weigh more than 20g, it will cost 68p to send it to France by airmail. If it weighs between 21-40g, it will cost £1.00. The current price of a First Class stamp in the UK for a standard letter is 46p.
However, the Royal Mail has announced a rise in postage prices that will go into effect on 30 April 2012. A First Class stamp for a standard letter will rise in price from 46p to 60p; a Second Class stamp will increase from 36p to 50p.
Therefore, as of 30 April 2012, it will cost 87p to send a standard letter via airmail to France, provided weight is 20g or below; for standard letters with weight of 21-40g, the cost will be £1.28.