The employer may wish to withdraw an offer of employment for a number of reasons, for example, because:
Its business requirements have changed.
It has unexpectedly received information about the applicant which casts doubt on the desirability of employing them.
One of the matters on which the offer was expressed to be conditional, such as receipt of satisfactory references, has not been fulfilled.
You could ask the potential employer the reason for the offer being withdrawn. If you believe that their reason could be for a potentially discriminatory reason, i.e. because they became aware you suffered from a medical condition and did not want to take you on because of that, then you may potentially have a claim for unlawful disability discrimination, however you would need evidence to support your allegation.
If on the other hand, the employer’s business requirements have changed, subject to your potential employer paying you the notice you would have been entitled to receive had you started work for them, then that would be the entirety of your claim.
Unless an employee has previous relevant continuous service (e.g. with the same or an associated employer) the employee will not have sufficient continuous service to bring an ordinary unfair dismissal claim. However , there are exceptions, e.g. claims for automatically unfair dismissal, where an employee does not need to fulfil any qualifying period of service to bring an unfair dismissal claim, i.e. if the reason for the offer being withdrawn is a discriminatory reason.