How’s it going man ? Based in the uk as well. Not a lawyer but as far as I’m aware and with every NDA agreement I’ve had, as soon as the project went live I was able to share images etc that I’ve worked on for said client, this includes companies such as American Express, Aixsponza, Amazon, even the project I’m working on right now for a bunch of small toys. Never been an issue.
take a look at the terms of the NDA usually they’re pertaining to disclosing information of any kind about the project before it’s released to the public, unless it specifically states other wise e.g you’re expressly prohibited from sharing any images or videos etc relating to said project publicly ever, then I don’t see an issue.
Although take this with a pinch of salt as I’m not qualified to give legal advice, you could try asking on one of the uk law sub reddits.
Could just be that Client A doesn’t want Client B to know/find out that they subbed out some of the work for said project to someone else.
Regarding your other question about using branded content for personal portfolios, again shouldn’t be an issue, as long as you aren’t monetising it, e.g selling digital prints or stills of a 3d Bart Simpson for example, or anything Nintendo. Stay away from Nintendo 😛