To answer your question directly, Zen have no FUP. You have a set amount of bandwidth a month and you can do what you want, you aren't limited in any way...
No port blocking
No traffic shaping
No throttling
No P2P restrictions
Nothing
I had two lines with them and I have switched both this month for O2 simply because I can get LLU and much cheaper than Zen, otherwise I would still be with them.
I was with them for four years, had to call technical support twice, once for a quick setup question, once when something wasn't working properly. Both times I got straight through to a technican in Rochdale who knew exactly what they were talking about, both times it was sorted instantly.
The best bit about ringing them is the phone traffic monitor on their website so you can see when the phones are busy so you can avoid hanging around in a queue!
You won't be disappointed with them if LLU with a decent provider (Be/O2) isn't available in your area.