I agree withTwinAisle's suggestion that Eastern are the most likely but, as he points out, the fares will be expensive.
In hindsight it is probably a pity that bmibaby ever went to CWL because Flybe would surely have seriously considered setting up a network from there not dissimilar to that from SOU and, increasingly, from EXT.
Their aircraft sizes are probably more suitable for several low cost routes not flown by baby out of CWL because the 737 is too big.
Flybe might yet make some sort of appearance at CWL of course and Aer Arann seems keen on building up routes from there, so who knows?
As for Air Southwest, they currently only fly within the UK and to the Channel Islands. I hope they will expand into the near Continent and Republic of Ireland.