The ones where you reasonably believe your customers are based, and where your employees are based.
Lets be honest, 1% of your customer base using a VPN is not going to cause you issues, unless those people are uploading something that would cause the state to act (ie CSAM, fraud, drugs, terrorism, you know the big four.) Given that this is the ICO, and nor OFCOM, we know its to do with GDPR violations, not moderation.
its not like the ICO just sent an email saying "lol you're being fined, bye". They will have had a series of communications, warnings asking for reasonable changes, time lines for change.
The ICO has discovered that Imgur are breaking GDPR in a fairly big way and in a way that can be easily detected by an understaffed and over worked semi-independent organisation.
moreover breaking GDPR in a way that is obvious enough in a court of law[1], bearing in mind that the UK, just about has a working independent and largely neutral judiciary that isn't easily intimated into doing the governments whipping.
[1] the ICO doesn't tend to be showy.