I dont see more then one 120 product on the draytek.co.uk site.

But, what you need a device to connect the WAN2 port of the 2920 to the internet. The Draytek 120 is ok for an ADSL/ADSL2 connection but, if you have something different like cable you need something else.
If you use an ADSL connection, maybe first check with the provider, what they recommend. If they have a modem (not a router), which works with an ethernet port like the 2820 has for WAN2, it is also a good choice to use this, because, if you ever have trouble, your provider cannot tell you it is the fault of the modem you are using.

Thanks.

I dont see more then one 120 product on the draytek.co.uk site.

Me neither. But Googling for a supplier, I found these:
http://www.amazon.co.uk/DrayTek-Vigor-120-external-Ethernet/dp/B0032Q120Q
and
http://www.amazon.co.uk/Draytek-Vigor-120-ADSL-Modem/dp/B003C1LYHC/ref=pd_cp_computers_2

This is where my confusion arises from. Are they, actually, one and the same thing, do you think?

If you lookup the 120 with the "B003C1LYHC" code,
http://www.amazon.co.uk/product-reviews/B003C1LYHC
people tell that this might be not for the UK market.