You will not find anyone who will do that in the UK - not no pass no fee anyway - since examiners are (in principle) independent of the flying schools. If you have been trained by a particular examiner you may not be tested by them, and anyone can have an off-day so it would be a bold school indeed that offered such a guarantee. Your instructor will have a good idea if you are ready but if you are still unsure fly with somebody else and see if they agree.