The problem with FPTP is that you can't just vote based on policies.
You also have to consider who is most likely to win the seat and whether you have to vote AGAINST who you don't want rather than FOR who you do want.
CGP Grey explains it succinctly, here: https://youtu.be/s7tWHJfhiyo
I haven't seen Gordon Brown in a long time, I thought he was Steve Coogan playing a character at first!