Hi,
HR for a security company here, so night shifts and WTD are pretty standard fair.
With regards to the WTD - most companies will have you sign a waiver nowadays, even if they don't intend to use it. Just a thing - it's not enforceable mandatorially in the UK - yet.
If he has signed, he can withdraw his consent but it will take three months for it to go through and you might want to ask what sort of impression it will give. For what it's worth, we recently suggested all our staff changed rotas so they did no more than forty-eight hours a week and they all said no - they want the hours because they want the money, so don't assume it's the company who are the issue, it might be colleagues.
With regards to days off - the law states one day off in seven - but it, like most things, is an average. If he does six or seven weeks without a break, then there's a problem but you've nothing before then, I'm afraid. Sucks, I know!
As for shift times - there's a requirement (that isn't breachable) of a minimum of 11 hours between shifts, so a finish at 6 to start again at 6 is perfectly legal - and actually quite the norm for some industries - like mine.
The trouble with the laws regulating shift work is that they also affect those such as Nurses and doctors, so no-one's willing to tighten them down too much in this country for fear of collapsing the NHS!
About the only thing you're husband can do, is ask for a change in his pattern under Work-Life balance rules. Just be aware that if he drops hours, he'll drop holiday days most likely, and that some industries, like security, aren't set up for Monday-Friday 9-5 so he might find himself 'requesting' his way out of any possible work!
HTH!