At company level, company agreements on working and employment conditions are concluded between employers and employees. It lays down minimum conditions of employment and national employment standards continue to apply. If an employer uses a registered agreement, the bonus does not apply. I would suggest an alternative that has not yet been mentioned in other answers: you can make a counter-offer for the deferral of payment. In other words, ask for an agreement under which the company owes you your entire salary, but they now pay you the amount proposed in their offer and the amount remaining one year from the date it should have been paid. Your company can get £X turnover for the maintenance of software Y. Most of the time, a company would lay off some of its employees to stay in the water. Our management saw that the situation was external and temporary and that there was no reason for the company to return directly to normal after the situation had resolved itself. Instead of layoffs (which can take a long time before they recover), my company has implemented a temporary 10% pay cut for all employees. The overall impact on the final result was the same, we were always able to fulfill all the commitments during this period, it was easy to rebuild it later and everyone kept their jobs and benefits. The only difference is that you are a volunteer, but that is probably because you are in Britain and not me.
They might ask you to sign something to confirm your agreement, but even if they don`t, it won`t stop an agreement from being legally binding.