Here is an important Supreme Court ruling on burdon of proof. In summary, the claimant must be able to convince a judge that they had suffered detriment due to a protected characteristic. If a claimant is unable to demonstrate the link between detrimant and the protected characteristic, the claim will not proceed. If they can demonstrate the link, burdon of proof switches to the employer to proove that discrimination had not occurred (decisions were not motivated by the protected characteristic).
Here is a link to the People Management summary of the Royal Mail Group Ltd v Efobi [SC 2021]