Transcreation needs a long term commitment to both France and its media culture and to the British way of life and language. Hiring a young gun is attractive; until they very quickly run out of ammunition. Experience is what you need first, not how fresh faced she/he looks. You need a French copywriter so before hiring one ask yourself when in London: does she/he listen to la matinale de France Inter? (highest audience for a early morning radio show) does she/he watch BFM TV?