On Now Broadband, valid yes, static no.
But we get along fine with one that varies, and sometimes we need it to, when some site or other decides we aren’t in the UK.
So why static?
If it’s just you, then a VPN is the safest option; otherwise, you will be running a server open to infiltration from all sorts of bad hats unless you lock it down pretty thoroughly.
Here’s a good article on the topic and your options here:-
You will notice that Now isn’t on the list of ISPs who provide static addresses, but you have the option of DDNS or using a VPN.
The Now router is unlikely to support DDNS, though if you can find a VDSL router that supports this, and in addition supports Sky/MER 61, you could use that.
And it would be well worth talking to Now Broadband about this intended usage, to make sure they will be comfortable with it, and can resolve any gotchas you might meet along the way.
However, selecting an ISP who can provide a static IP address in the first place might be a better answer all round.