Thank you all for your replies.
I was considering Zen (business, unlimited fibre 2 or unlimited fibre office), but after reading the last two replies I'm once again stuck. I look at Plusnet and see a bit of BT in it (despite the fact they're actually operating nothing like BT), but what mostly concerns me is the longer contract time vs Zen, implementation of traffic prioritisation and some comments I've read about possible congestion. And then I look at Zen, I see a company that seems to have no traffic prioritisation implemented, cheaper call rates on business line rental, though on the other hand I haven't found too much user feedback to know for sure whether or not the network has a good amount of spare capacity to not cause congestion issues during peak times. I imagine for both Zen and Plusnet that these problems don't really exist and I'm concerning myself over nothing.
I haven't phoned both companies yet to confirm the pricing and availability of multiple static IP's, but I will be. My first step is to get a MAC out of BT Business, which is the easiest step I hope. The hardest step is to choose between Zen or Plusnet.
Any more opinions are welcome, thanks for the input
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EDIT #1 (30/06/2014 09:58): Called BT Business, after a short wait I managed to get the request for a code made easily, been told I must wait up to 5 working days and to regularly check my email for the MAC code. Also noticed a reply from Zen Internet in my spam folder that confirms they offer multiple static IP's for both residential and business packages, and there's no extra charge. BT Business have offered me half price 12 months if I renew my contract, though I imagine it would mean another 24 month contract, so after 12 months would return to the normal price. While it's a nice offer, I don't think I'll take BT Business's offer up.
So far it looks like I'll be picking Zen, will keep everyone updated for those curious about what I do and how I managed to get on.