I've been with BT Internet for some years now, and recently connected to ADSL. I did have initial problems with the line (at the limit of distance - limit is about 5.5Km line distance from the exchange) and my computer (interaction between KT133 chipset on the VIA motherboard). However, the BT engineers have been brilliant in getting the problem solved. Now using ADSL daily (connected for v. long periods without problems) and no complaints. Upload speed about 280Kbs, download 576Kbs.
Pre-ADSL was with BT Anytime (£16 per month and no call charges for H24 connection). The only drawback is that it automatically breaks the connection after 2 hours. That's to stop people staying connected H24. However, you can re-connect immediately with no problem. Difficulty only arises if you are trying to download software where the time to download is more than 2 hours.
On balance, I would go for BT Openworld, but if you wish to e-mail me and tell me where you are going, I may be able to find out some more info for you - I have a friendly BT engineer contact.