Very much depends on you, your likes and abilities.
I would spend it on multiple punts, but mixed in with one expensive SP that is particularly fitting for my needs. So, something like 1x£400 Plus 4x£200.
However, this is being aware that I could never get the most out of some of the long duration, multi-girl sessions that some of our members describe. As much as I enjoy reading about their exploits, I know that after 30 minutes I would be exhausted and sitting in a corner just watching. Similarly, there would be no point in me going on a tour, either UK or abroad, as I need days to recover between punts. If I was half my age, I might have a different approach.