Mullvad VPN may be a good choice and the good thing about it is usually fixed price of 5 pounds even if you buy for a single month or for a decade 
You get a discount as well if you pay with crypto
However, I just wonder why they aren't as popular as Nord or Express VPN? Do they (express/nord) offer extra services etc 
One of the reasons is because mullvad doesn't do aggressive marketing like nord and express VPNs. TBH, I've seen these two vpn providers everywhere lol. For instance, I would see them sponsoring the most random video on yt lol-- I do not mean buying ad space on yt.com but sponsoring a youtuber's video.
Mullvad doesn't do paid reviews, KOLs and affiliates. It's a part of their business ethics which I have big respect for. This may be anecdotal but, I wasn't aware mullvad existed until someone in the forum mentioned it.
I do think differences in features does play a part as well. For instance, mullvad doesn't offer dedicated IPs like nord.