May I ask why? It is in fact one of the policy reasons behind the law of surrogacy in the UK and the reason why we do not (generally) sanction surrogacy arrangments involving payment beyond medical expenses.
People aren't allowed to receive payment in this country in return for donation of organs either, so what point are you trying to make?
Elton and David's surrogate is in the US, so UK laws don't apply anyway. We also don't know whether or not the mother of the children has contact. She may well do.