The 2850 appears to have the ADSL/VDSL modem integrated into the router. From the draytek link given it also suggests the maximum speed on VDSL (BT Infinity) is 50meg. I dunno though if that is based on the BT specs when the site was written, and their leter services weren't known about.
The 2920 which we use has the ADSL connected into WAN 1 and Virgin into WAN2. Oddly WAN 1 is only 10/100 ethernet speed, whilst WAN 2 is gigabit. To get ADSL we needed a separate ADSL modem, the Vigor 120 which is actually controlled from the 2920. VM's superhub was simply connected from one of it's LAN ethernets to the WAN ethernet of the 2920. It works and we can get 48Mbits + on the 50 Meg service. But then we also don't make use of wireless and that, especially if you need dual band, could be a factor against the 2920?
We started with the ADSL connection as our prime service, and then gradually VM became the faster speed available. I haven't changed any physical connections, simply some of the bandwidth and routing configs in the draytek control panels. All works fine so you should be able to easily swap your prime service ADSL/ Cable / FTTC as an when provided they support an ethernet connection from their modem to your router.
It's probably worth checking the
comparison chart to see what options are best for you. The real risk is that speeds of broadband are starting to exceed the WAN side's of many routers