As I understand it, the reason for using a *combined* Router and ADSL Modem is that it's not as simple as 'daisy chaining' an ethernet ADSL Modem and Cable/DSL Router.
This is because, in the UK, ADSL uses PPPoA protocol (PPP over Asynchronous Transfer Mode) and generally Cable/DSL Routers only support PPPoE (PPP over Ethernet). In countries such as the US, where their DSL uses PPPoE, this isn't a problem.
What you could investigate is using a conventional ADSL Modem/Router and plugging into it a Dual Band Wireless Access Point. You'd then connect to the separate W.A.P. instead of the router's built-in one.