It’s completely legal to stream most types of adult content in the UK and most of the world. There are exceptions, such as ‘grossly offensive, disgusting or otherwise obscene’ material, or any that depicts ‘life-threatening injury’, but you’re unlikely to find this unless you’re actively seeking it out.
There have been several attempts to restrict access to pornography, both in the UK and the US, however, which is where much of this uncertainty comes from. The British government has, on multiple occasions, failed to pass bills restricting access to adult websites. It’s worth noting that even if these had become law, porn would have remained legal in the UK but consumers would have had to verify their age either in-person or via a potentially-unsafe online system.