They have a complaints procedure page on their website giving the contact details of their Compliance Officer. Bang in a complaint and if the outcome is unsatisfactory go to
Financial-ombudsman.org.uk.
Like macdo, I would be interested to hear why your claim has been rejected. If you were never going to be eligible for benefit under the policy for some reason, then it may well be a misselling situation.
If the policy is void then you may get a return of premium plus interest.
Good luck.