A contract is a legally binding agreement between parties. When Robert Cook signed his employment contract with Sheffield University he agreed to its terms, including the conditions for terminating the contract, which are very similar to the terms in my 1986 contract with Nottingham University and to those of many other institutions' contracts.
I assume that he would not consider it reasonable for the university to break any of the terms in the contract and would consider suing them if they did so. So why does he think it is reasonable for him to do so? Toys and pram come to mind.