Soon after I started posting on this site there was a woman posting, arguing that an employer could place this condition on employees – I argued the opposite, that any UK court/tribunal would find this an unreasonable restriction on freedom.
The care home could advise that as a result of your refusal to accept the change in terms of employment, they would sack you. Once sacked, your only recourse would then be to take the matter before an employment tribunal. If you have been employed by them for less than 2 years, this option would not be available to you – unless you could argue the matter based on sex discrimination (or some other criteria allowed within this period).
Personally if this happened to me I would tell my employer to stuff it (and take them to an employment tribunal), but then I’m in the luxurious position that I don’t need a monthly salary to survive. What with the UK economy about to take a serious dive with many millions unemployed, you might decide that your only option is to accept the terms.