You are correct in assuming the former - unless they've changed from the last time I looked, the ADSL modem supplied by ntl:freedom is USB only. You would probably be best off buying a router with an ADSL modem built-in, as it's likely to be cheaper!
By the way, there's more information (including direct phone numbers) for ntl:freedom at their website
www.ntlfreedom.com 
EDIT: Scrap that, it seems they've finally merged Freedom with virgin.net. More info at the website above - but again, virgin.net provide a USB only modem by default - although I think you might be able to pay virgin.net for an Ethernet one...