FlyBe also operate from Birmingham to Paris Charles de Gaulle. The cheapest flight on Friday 21st is the early one at 0630 (arrives 0900), with an inclusive fare of �57.72. The flights at 1030 and 1310 are probably too expensive (�142.72). The 1725 flight (arrives 2000) is priced at �72.72. Returning on the 23rd, the 1410 flight (arrives 1435) is dearest, at �107.21. The flights at 1620 (arrives 1645) and 2035 (arrives 2100) are priced at �82.21.
FlyBe's services from Manchester to La Rochelle don't help, because they don't operate on the right days of the week. Their services to Rennes have suitable times but the prices are quite high. The outbound service, from Manchester on Friday, is at 1005 (arriving 1255), priced at �132.73 inclusive. The return service, on Sunday, is at 1320 (arrives 1410), priced at �86.09. Return train fares, from Rennes to Nantes, start at �23 but you need to be aware that the first train on Sunday doesn't arrive at Rennes rail station until 1151, which doesn't leave you much time to get to the airport.
FlyBe offer plenty of flights from Manchester to Paris Charles de Gaulle but they're all quite pricey. (From �102.27 to �132.27 outbound and from �97.66 to �197.66 return).
Jet2's flights from Manchester to CDG are cheaper. The outbound flight, on Friday 21st, leaves at 1300 (arrives 1530), with an inclusive fare of �93.00. The return flight, on Sunday 23rd, leaves at 1620 (arrives 1650), with an inclusive fare of �70.21.
http://www.jet2.com/
Summing all that lot up, it looks like your best option (if you don't mind hiring a car) might be the Birmingham to La Rochelle route. Otherwise, East Midlands to CDG might be best.
Chris