I've been told that a "one-trick pony" is an American saying. What is the British equivalent for it?
Consider the following, which looks at the issue from a different perspective: instead of saying it can only do one thing, this speaks about wanting to do more than just one thing.
have another/more than one string to your bow uk to have more than one interest, skill, or resource that you can use if you need to: I enjoy my work, but I'd like to have another string to my bow in case I lose my job. - Cambridge Dictionary