Will I need to buy a modem for my Airport Extreme or can I just set it up out of the box, / is it a combined modem and router? And could I use my Business Hub (2701) as a modem using the fibre port on it?
All you need do is attach an ethernet port from the WAN ethernet port on the Airport extreme to an ethernet port on the 2701.
If you have fibre you may actually be able to take the 2701 out of the equation. From the specs it states that the Airport Extreme can do radius authentication, if this is the case it should be capable of PPPoE authentication. This being the case, simply remove the ethernet cable that plugs into the fibre port on the back of the 2701 and plug it into the Airport Extreme (so 1 end is in the white modem and the other is in the Airport Extreme). Then set PPPoE authentication up on the Airport Extreme remembering to set the username and password to those given by BT (the username will be similar too C123456@hg12.btclick.com).
If you have a device behind the 2701 that can do PPPoE authentication you are simply complicating matters by putting it behind another router.
Airport Extreme does not have a modem. I don't know anything about the 2701 but in a normal ADSL modem setup, which may be the same, have that connected to the BT line and then use an ethernet cable from one of the ports to connect to the WAN port on the Airport Extreme. Apple Support pages has good information on how to set up networks using the Airport Extreme. There are several options which will include allowing you to run more than one network.