I think it is a cultural thing. The representation of black women in UK media is even worse than in the US. There aren’t many dark-skinned black women in UK media and the ones that exists are typically either playing mammies or masculine roles. Desirable or feminine roles are usually given to light-skin actresses.
A major difference between UK and USA is there is greater pressure to assimilate in the UK vs USA. So black people are more likely to marry white people in UK. When I lived in the UK, I saw very few young black couples and the one’s that did exist were usually Africans.
Therefore, because there is even less black men dating/marrying black women in the UK coupled with the terrible dark skinned media representation in the UK, makes dating as a dark skinned black woman in the UK very difficult.
The difference between UK and other countries with a non-black majority is that they are less likely to consume UK media and therefore more likely to consider dark-skinned black women desirable.