If you want to connect your amiga directly onto the broadband network, then BT is a no-way! Even if your amiga has USB ports (Subway?), there are no drivers for bt's crappy modem (there are no drivers for linux either). The best you could do is connect it on a pc and use it (the pc) as a gateway to your amiga.
It gets much simpler if you go for cable broadband (telewest, ntl). The cable modems usually provide a lan port, apart from usb, so you can connect your computer on it and access the modem through your web browser (the address is usually 192.168.100.1) and treat the modem as a lan router(note:lan router != switching router). This way you only need a lan interface (PCMCIA, ZORRO 2, PCI).
Telewest now gives 768 kbps for £27.99 PM. I think it's a good value for money.