The idea of a DSL modem using a USB connection to the computer was a kludge put together by the industry several years ago to "simplify" the installation process for unsophisticated customers at home. While corporate computers generally had ethernet interfaces, most home computers at the time did not. But they almost always had USB ports. So the DSL modem makers came up with a product that did not require home customers to open their computers and install new hardware and drivers (the ethernet card).
The world is different now. Most new home computer have ethernet ports because broadband and multiple computers in the home are so common. Thus, the DSL industry doesn't push the USB modems so much any more. Unfortunately, you are caught in the middle with an obsolete piece of equipment and you can't add anything to your configuration. There may be some odd broadband routers that take a USB input, but they may be hard to find, may have specific installation requirements, and who knows how well they will work. In essence, they are really just another layer of kludge attempting to make the original kludge work better. You will probably be better off starting over and doing it right, with ethernet rather than USB.
I agree with Michael's suggestion - try to get your provider to replace the old one with a new model with ethernet. You will need to work with them rather than trying to buy something on the market as there are a number of variants of the DSL technology in use around the world, and you need to have exactly the right kind of modem to communicate with the particular equipment in the telephone company switching center. Even if you know the brand they are using, there may be something peculiar about the internal settings that you would have to find out from their technical support team.
Once you get a new modem with an ethernet connection, you might be better off not buying wireless equipment in the USA unless you know it will be compatible at home in the UK. The radio frequencies and channels that are used for the wireless link are regulated by governments and it may be that there is a different division of the channels in the UK. (I don't know the details of our regulations vs. yours, but be aware that it could be a problem.) If so, your USA-bought equipment could cause interference with nearby radio equipment and it wouldn't work with any other WiFi equipment over there. Thus, the bargain price would turn out to be a complete waste.
As for your question about DSL modem/router/WiFi combination equipment - yes those exist, and are becoming more common. My personal opinion, though, is don't go there. As I noted, the DSL modem section of it is very specific to your service provider, so you won't have many choices. If you go the next step and tie the WiFi together in one unit, you will have few choices of wireless types and features. For better or worse, the wireless industry is constantly changing and improving. Furthermore, despite the apparent standards, not all brands of equipment work well together. If your WiFi base is in the same unit as your DSL modem, you are stuck with what it does now, and no means to upgrade to newer faster speeds or improved security features in the fututer. And if the wireless part breaks or doesn't work right with the client device in your laptop, you are out of luck unless you can figure out how to turn the WiFi off in the modem and then install a separate WiFi router that meets your needs.
Good luck with it and enjoy your visit here in "the colonies."
dw