With Zen you will see a lot less by way of traffic shaping and other throttling than you will with PlusNet.
I've never used them myself so can't comment much further, but I know people who do use both and I get the impression Zen is noticeably more reliable. The people I know who are on PlusNet are not unhappy though, so they can't be particularly unreliable (just Zen might be a little better).
You don't have the option of Be, which would otherwise have been what I'd suggest you look at.
A&A might be an option if you use very little downstream bandwidth during weekdays between 0900 and 1800, though they don't do a phone calls package so you'd need to pay line rental and calls to BT or similar. Their cheapest arrangement on 20CN is £19.40 which would get 1Gb peak and 50Gb off (or 0.5G peak, 75Gb off and so on - you can spread it how you need to). That plus a line and calls from BT would be something like £35 so you could buy more bandwidth with the £7 difference between them and Zen. When you get access to 21CN in October, the amount of bandwidth you get for the same price will double if you switch (2G peak / 100G off-peak for £19.40). See http://aaisp.net.uk/broadband-prices.html
for a little calculator and full price list. Uploading is not metered at any time so the bandwidth quota you pay for is only affected by downstream traffic, there is a spot in the night that is far cheaper (useful if you can schedule big transfers to happen while you sleep), and you can carry a month's under use over to next month (or if you go over one month you can take the extra off next month instead of paying more). And completely unshapped or otherwise messed with, they don't even use the IWF blacklist. Slightly more complex to price up then other ISPs offerings, but flexible and (unless you need a lot of transfer M-F/0900-1800) good value IMO.
I use A&A and I've been pretty impressed so far. They are very techie friendly (native IPv6 available if you want it, multiple IPv4 addresses with full RDNS delegation, lots of stats on the control panel interface, ...) which matters to me, though from your post I guess that isn't something you care about. Their support was good at the start of my FTTC account, when I had some odd line trouble.
Long story short, I'd probably suggest:
1. PlusNet if cheap is particularly important
2. Zen or A&A for something a bit more reliable (though PN is probably reliable enough) and unshapped
3. Zen if the connection might be used much during the traditional working day, A&A otherwise
Current ISP: Andrews & Arnold (AAISP, aa.net.uk) via FTTC at ~36Mbit down & ~10Mbit up, joined July 2011.
Previous setup: Be Pro with UploadPlus (ADSL2+, AnnexM), 12ish Mbit down, 1.6 up, happy customer for ~2.5 years.