A couple of points about this. The kill fee may exist in a contract, but I've seen a publisher say effectively 'so sue us.'
More importantly for UK based authors, the Society of Authors has a brilliant free contract checking service for members. There is a fee for membership, but it's well worth it. To be a full member you have to be published, but you can join as an associate member as long as you have an offer, and make use of the service.