You don’t notice them as often in UK stores as in the US because they’re not generally in the child safety section. They tend to be off with electrical supplies and tools. They’re also often smaller and their intended use is often less obvious. However, switch locks are available for the same uses as in the US: particularly keeping freezers, pumps, generators, and medical equipment from being turned off accidentally. Also as in the US, available in different styles and materials.
Here is a popular one that is generally sold as 4 switch guards for £5. (The black oblong is the switch lock in this photo, but they also come in white,)
https://www.amazon.co.uk/Switch-switch-prevent-switches-leaving/dp/B00I3GLL9W
There are several available at Amazon.co.uk.
There are also UK community members who have designed switch covers that fit over an existing switch and then you put one of the two battery powered switches that I’ve mentioned on top of that. These can look very nice. Here’s one:
