The Working time regulations 1998 dictate a maximum of 48 hours a week over a 17 –week period, with night workers to average no more than 8 hours in every 24 hours, again calculated over a 17 week period, the right to opt out of these restrictions exists as does the right to opt back in. My understanding is that your husband and his colleagues have opted out of the Working time regulations and their Contracts of employment reflect this.
If their employer wishes to vary the terms of the Contract of employment, from what you say this will probably be by agreement, it could be achieved under section 4 of the Employment rights act 1996.
If it is a “take it or leave it” attitude on the part of the employer rather than genuine consultation and your husband was dismissed he would need a one year period of employment to take the matter to an Employment tribunal for unfair dismissal.
In practice I understand your husband and his colleagues do not want their hours increased, but understandably in the present conditions do not wish to be dismissed. There is strength in numbers and if your husband is a member of a trade union he should speak to them immediately, if he is not a member of a trade union all those employees who will be affected by the variation should present a common front to the employer and demand meaningful consultation.