History. Mainly because I didn't have a clue what I wanted to do and it was my favourite A Level subject. It was absolutely the right decision, there were loads of career options open to me at the end of it. I've ended up in Finance but others from my degree classes are in the civil service, marketing, academia, HR, the museums/archives sector, management consultancy, running their own businesses, and loads of other things as well as the obvious "history teacher".
I think a broad subject is absolutely the way to go if you don't know what you want to do. Transferable skills sounds like a trite 00s catchphrase but it's true. I've been recruited and promoted loads of times above people who did degrees specific to the area I work in, because it's the sort of area where you have to undertake professional qualifications anyway which always supersede the undergraduate degree.
However, if you're looking to retrain then you'd presumably have an idea of the area you want to go into and can find out the best subjects and routes into that - which may not be studying at undergraduate level at all.