There are several options here - depending on what you wish to achieve. What follows below assumes you now want to rethink your decision to employ - which may be wrong, so please advise.
I wouldn't get hung up about the 'discrimination' bit. An employee has to have 12 months continuous service before he/she can bring a claim to an Employment Tribunal - so that is never going to be an issue UNLESS this employee could find a way of fitting into the 'Autmatically Unfair' categories, in which case the 12 months service is not applicable. See here for more on automatically unfair (down the page a bit).
http://www.direct.gov.uk/en/Employment/RedundancyAndLeavingYourJob/Dismissal/DG_10026692
All of the above assumes that the employment contract was initiated. Until the employee turns up on the first day, I believe that it hasn't - but I'm not prepared to put my neck on the line to say that. Let me explain further.
There is a difference between 'wrongful dismissal' and 'unfair dismissal'. Wrongful dismissal is a breach of contract, demonstrated in front of a judge. Unfair dismissal is a situation where the employer fails to satisfy the requirements of the employment legislation, demonstrated in front of an Employment Tribunal - but to be able to make a claim, the employee has to have 12 months service - unless the claim is for an automatically unfair reason.
Whilst I doubt that the employment contract has even started until Bloggs shows up - I'm not prepared to advise you to rely on that route - so I suggest you just check that you couldn't be caught later under the 'Auto-Unfair' rules.
Wrongful dismissal involves a civil case. You have written to Bloggs and confirmed that you will employ him from 1 October and the the notice peri