I'm moving to Zen as they've long held a reputation similar to Nildram, but are currently a lot cheaper (only £1 less on the basic unmetered 512kbps package, but with free static IP blocks, where Nildram charge £3.50 for a single IP then £5.88 for a block of 8). The price difference wasn't enough to move, but Nildram's stability has been shocking for the last few months. It might be worth you checking out ADSLGuide - look at the 'fastest ISPs' chart, then poke around the forums for some of those ISPs.
I don't have any flashy requirements (like fixed IP, or the need for decent upstream speeds or even email/webspace) so what I can't decide is whether a basic package whose only limitation that might affect me would be a bandwidth cap of something like 1 or 2 Gb a month, or if it's worth the extra few quid a month for an uncapped service. I don't do any internet gaming, download movies or whatever. The only big downloads I do are new software packages (I'm not doing anymore OS updates since the last one killed my laptop for a bit). I guess the question I ask is, how much bandwidth does a fairly normal amount of web browsing and the odd bit of ssh and scping actually consume?
Oh, and the other issue, of course, is that my only computer is a Mac, so a Mac-aware (if not outright friendly) ISP would be a bonus. In any case, I'll want to purchase an ethernet modem that's compatible with my wireless box.