My computer is connected to the internet via the
phone line and the place where I live has just got
broadband. When I look to see if I can use this I am
told that it's not available for OSX Tiger.
Virtually all UK Broadband can be used with a Mac running Tiger, regardless of what is said on the ISPs website. They may not support OS X but it will work.
Check out the following website.
Reviews of equipment
http://www.adslguide.org.uk/reviews/
Comparisons of ISPs
http://www.adslguide.org.uk/isps/compare.asp
I would suggest going for a wires only deal and purchasing your own equipment.
Do not get a USB modem unless you are going to only use it to keep a door open or similar... 😉
As most ADSL Modem Routers are setup through a web browser you should be fine with a Mac.
Personally I use plusnet and Netgear ADSL Modems. One is a DG834G combined device and the other is a DM602 Modem Router connected to an AEBS.