I started recommending BT to business customers a while ago. They have some annoying anti spam policies (mail relay whitelist where they check on WHOIS and compare postal addresses), extra cost for static IP etc, but they seem to be OK, not too over subscribed (i.e. it's fast enough), the customer support on the business side is good enough if you know what the problem is yourself and are able to explain things. They're good enough if you don't, but the problem I had with Demon was when they transferred their support to India or wherever, they were useless in the former situation, somewhat condescending and almost totally unable to escalate matters to real technical people, much like I encountered with Wanadoo (not my own, on behalf of customers), hence why I'm quite happy with BT's business broadband stuff. I think though that the residential support might be India based? Not sure. I use the 0845 600 7020 when I need to call them.
I'm with Be personally and have a few customers who I've directed that way as well, maybe more from now on. I worry they'll be oversubscribed soon with their unacceptably low prices and O2 also offering out Be's network at even lower prices. Their support has been fabulous with me during some slightly weird phoneline/number circumstances on my behalf. Very good to deal with.
Oh and Be's multi-static-IP stuff isn't ideal. They bridge your router with their network and allocate you a few IP's on their subnet, rather than giving you your own little subnet. So you can't for example have a nice router firewalling the static IPs nicely unless you do something like 1-1 NAT for each IP and use a private IP address on your machines that would otherwise have had the public but firewalled IPs.