I don't have any direct experience of a combined adsl modem and router, having bought a wireless router from dell when I bought a laptop from them recently.
I then discovered that my usb adsl modem wouldn't work with it and bought a Netgear adsl modem with the required ethernet ports.I have not been able to find a wireless router which works with a usb modem.
The two separate items cost much the same as the combined model. The modem has worked faultlessly since installation and other people I have spoken to recommend them. For the combined model see click here
I should point out however, that this is not a cheap option and you may well find that your plan A would be a good compromise. Also bear in mind that if you sign up for BT Openworld there is minimum 12 month subscription. So far the only isp I have found who do not have a minimum subscription period are Zen.
I would be glad to hear of any others and people's experience of them because I am researching the exact same scenario as TomMK for my student son and his friends. They are also looking for cheap Sky digital and cable is not available in their area.