I mean, the low case rates could be due to the lockdowns, not sure about mortality rates unless you're comparing it to places where the ICUs have become overwhelmed.
The same website the OP links to, which is an Australian site, has a page that includes the text, "Vitamin D deficiency may be a greater health concern for people with naturally very dark skin, as it is more difficult for people with this skin type to make vitamin D.".
It's unclear why an Australian site would be going into the question of who is "suited to" live in the UK.
Furthermore, this article by two Australian academics, published in an outlet considered leftist, is basically about why Australians have such high rates of skin cancer, and includes the quote:
Why us?
Most Australians (and Kiwis) have the wrong type of skin for their environment. Basically, through migration, our two countries have been populated by many people with fair skin whose ancestors come from much less sunny climates. Lack of protective pigmentation leaves skin cells especially vulnerable to the DNA-damaging rays from the sun.
During human evolution, our early hominid ancestors in Africa lost their covering of body hair and developed pigmented skins, presumably as protection against the harsh tropical sun. With subsequent migration out of Africa into Europe, the protective benefits of dark skin became less important for survival and were likely a hindrance to effective vitamin D production. There was selective pressure for less pigmented skin with more distance from the equator.
In contrast, those who migrated out of Africa to Asia, Australia and the islands of Melanesia were constantly exposed to sunlight. So they retained their protective pigmentation. This explains why the recent European migrants to the Americas and Oceania arrived in the “new worlds” with skin types poorly suited to their new environments. This was in stark contrast to the well-adapted skin of the indigenous inhabitants.
Bro, the exact website you quoted (not same page) has an entire section of information about melanin, ethnic groups, and vitamin D:
What is my skin type?
Skin types that are more sensitive to ultraviolet (UV) radiation burn more quickly and are at a greater risk of skin cancer.
All skin types can be damaged by too much UV radiation. Skin types that are more sensitive to UV radiation burn more quickly and are at a greater risk of skin cancer.
People with naturally very dark skin (usually skin type V or VI) still need to take care in the sun even though they may rarely, if ever, get sunburnt. The larger amount of melanin in very dark skin provides natural protection from UV radiation. This means the risk of skin cancer is lower.
Eye damage can occur regardless of skin type. High levels of UV radiation have also been linked to harmful effects on the immune system.
Vitamin D deficiency may be a greater health concern for people with naturally very dark skin, as it is more difficult for people with this skin type to make vitamin D.