ISDN is quite a bit more expensive than a normal BT line. BT Home Highway is a kind a pseudo ISDN which gives you two analogue lines for voice and two data lines for ISDN although its only possible to use any two of the four at any one time.
Another thing to think about is that to use ISDN for internet access would need a Router/ Terminal Adapter (Approx £110)
Also call charges on ISDN are roughly 4p a minute!!! But you can use BTs 0800 unmetered service on a 64k dialup.
Hope this helps a little.