I lived in South Africa for about 30 years, and black and brown widow spiders (known as button spiders in Africa) were common. We found them in the garden all the time. There are several species, and colour/marking variants. None of them are very big, but the females are a lot larger than the males and their fangs are quite capable of penetrating human skin. They're not generally aggressive, but the venom is a neurotoxin and the spiders are considered dangerous. Fatal bites are not very common in adults, but the effects can be severe and the very young, elderly and some others are at greater risk. Urgent medical intervention is required.
Widow spiders are normally found in warm climates, but I expect they could survive, for a time, in the UK providing they found a suitable place to live - they're web builders and don't roam very far - and a food supply.
