smallpilot
Think you should be able to get around £12000 for a STN based 737-300, smaller aircraft would not be a great deal less as you would be losing out with positioning costs. Always better to go from where the a/c are based.
Likewise LGW-TFS-LGW would only make sense for a Spanish carrier if operated in reverse and cost would probably be not too much different from a UK carrier. Approx £32,000 for 733 or £42,000 for 752.