Most DSL and ADSL UK broadband ISPs give USB modems with their services. Cable modems generally have an Ethernet port (they can have both USB and Ethernet so check carefully). To find out what kind of modem you have, check the connector and lead where the modem connects to the PC.

Ethernet looks like a regular phone jack, but larger. It may be built-in, like this:

or installed in your machine as a PCI card:

USB looks like this:

With USB the symbol below is always present, on the device, lead or computer.