KLM does not normally do affordable one-way fares, with a few exceptions - usually from airports where they are in direct competition with someone who does (no hope, ever, from my home airport, NWI).

I am only guessing... but it looks like perhaps they were carrying out a temporary experiment with affordable one-way fares on your route. GBP 400 one-way is what I generally expect to see with KLM - it'll be about half that if you buy a return and throw away the other half.