They only have to abide by the itinerary for the purposes of calculating any Delay Repay compensation. They clearly will find it impossible to get the OP to Preston Park in accordance with the itinerary.
That they may find it impossible in practice doesn't mean that their legal obligation to abide by the itinerary ceases to exist. They will still be liable for the OP's cost of provisioning alternative transport to Preston Park if Brighton is the nearest railway station to which they propose to deliver him (and assuming they inevitably refuse to provide the alternative transport themselves).
On a general point, it may be bad form to renege on a contract but it is not 'binding' except that if they are unable to provide you with the service you are entitled to a full refund without any admin fees. There is no provision in English law that binds a business to having to deliver a promised service regardless of circumstances. It's no different to buying an out of stock item on a website and subsequently being told 'sorry, we can't supply' and getting your money back.
It is not particularly helpful to compare the situation at hand to ordering goods for delivery online. In that example, submitting an order does not constitute making a contract, it is merely indicating that you agree to purchase the goods on the terms of the invitation to treat they have put forward by advertising the goods for sale on their website. A contract is only made once the supplier has agreed to the contract, taken payment and dispatched the goods.
With train tickets, as soon as payment is taken, I don't think anyone is disputing that a contract has been made. It is, therefore, trite law that the train company cannot revoke that contract once it has been made; only a change in the circumstances that is so substantial that it invalidates the basic premise of the contract would allow them to do so (e.g. unforecast extreme weather). In any case, even if such revocation were permissible as a matter of common law, the train companies have put conditions in place in their terms, which clearly indicate that it is not their intention to abdicate of their responsibilities as soon as things go wrong (c.f. Condition 28.2 of the NRCoT), and indeed that they guarantee the right to compensation for (most) qualifying delays (c.f. Condition 33).
Therefore, both as a matter of common law and through the terms on which the train companies have freely decided to sell tickets, train companies cannot get out of their responsibilities and liabilities in situations like this. Coming back to the situation at hand in this thread, the fact that road access to Preston Park station will be limited or nonexistent is entirely irrelevant. The key element of the test to allow for revocation simply isn't satisfied - there is no change in circumstances, since the railway still has a perfectly acceptable means of getting people in and out (the tracks!), and in any case the road closures and the marathon were known about by the TOC long before the contract was made. If they were so incompetent as to fail or forget to update their timetables in accordance with their intentions, that is (legally speaking) their own fault and certainly not the passenger's fault or problem.
