You could try the CAA. They have a form that should be filled out (CA 680) in the event of a birth onboard a UK registered aircraft. I expect that it is a very rare occurrence, as most airlines will not carry a female in the later stages of a pregnancy. My airline requires a "fit to fly" letter for weeks 28-34 of a pregnancy.
If an expectant mother were to go into labour, the baby would be premature. Time to get it on the ground.