I don't think anyone has mentioned the decision of the Supreme Court in Societe Generale v Geys which concerns when a contract of employment is terminated following summary dismissal, and has some interesting discussion of the general contractual position in the dissent of Lord Sumption.

Simplified we have a banker who under his contract of employment is entitled to a huge bonus if his employment is terminated before the year's end, but an absolutely ridiculously huge bonus if his employment continues into the next year.

His employer called him in at the end of November and told him he was sacked. After a bit of to-ing and fro-ing, his solicitors write back at the start of January to say they affirm the contract of employment, and he would like the absolutely ridiculously huge bonus please.

Under the terms of the deal the bank could have terminated his contract by making a (small) payment in lieu of notice, but they did not do this when they purported to summarily dismiss him (whooops!).

Under the terms of the deal to earn the absolutely ridiculously huge bonus the banker doesn't actually have to do any work, he just has to be in employment at the relevant time. 

A contract cannot according to (English) orthodoxy be unilaterally terminated (save in accordance with its terms) and I was always brought up to understand that a repudiation that is not accepted is a thing writ in water. 

requires the co-operation of the repudiating party, and (ii) the contract is incapable of specific performance, with the result that that co-operation cannot be compelled."

Under many contracts in order to perform your side of the deal you need the counter-party's co-operation. Under my contract of employment in order to earn my salary I need my employer's co-operation in order to be able to work. So I need access to the lecture halls, students allocated to me to supervise etc. If I do not do my work I cannot fulfill the condition precedent to my getting paid my salary. I can only therefore maintain an action for my salary if my employer co-operates in allowing me to work. If they don't, damages is all I'll get.

That does not mean that the contract of employment can be automatically terminated by my employer. That it is not may be illustrated by the fact that my employer can withdraw its repudiation of my contract at any time up until my acceptance. If termination was automatic, it could not do so.

The peculiarity (?) of Societe Generale v Geys was that Geys was entitled to the bonus without having to do anything for it. If he was employed he got it. So the co-operation rule enunciated by Lord Reid, when properly understood, does not apply.