There is an offence of Corrupt Withrawal of Candidature, under the Representation of the People Act 1983, which has been mentioned in the Daily Mail.
It sounds like that could apply but having just looked at it, it says,
"Any person who corruptly induces or procures any other person to withdraw from being a candidate at an election, in consideration of any payment or promise of payment, and any person withdrawing in pursuance of the inducement or procurement, shall be guilty of an illegal payment."
As it involves only of a payment or promise of a payment and not any other type of inducement I cannot see how it would apply.