day 1-4 Paris
5 Eurostar --- London
6 rent a car --- Windsor / Legoland
7 巨石陣 / Bath
8 Cotswold
9 Windermere
10 Cambridge
11 return the car
12-14 London
15 HK
Last August, we went to England as well. The itnerary is similar to yours but we also went to Cadbury at Birmingham. I think it is worth spending one more day at Windermere.
You may try Cathay Holiday. I agree that you should choose direct fight if travelling with kids. We have booked CX tickets to Germany (before they offer the holiday package), we are now booking hotels and trains.