This is a good read for compliance to building regulations when it comes to downlights in ceilings that enter attic space - http://www.aico.co.uk/firecap_loftcap.htm. Chapest ive seen these is here http://www.allaboutelectrics.co.uk/doc/12/vid/2447/AICO_THERMAL_LOFT_CAP.html
Volex are owned by Electrium based in Cannock near Wolverhampton. They also own such brands as Crabtree, Wylex and more recently Siemens. http://www.electrium.co.uk/index.htm. They are a very reputable company
I looked at the ACM100T fan and then found a Vortice Lineo 100 fan which had the same extract volume and was much cheaper (about £30). I got it from here http://www.allaboutelectrics.co.uk/doc/12/vid/2078/LINEO_4__DUCT_FAN_TIMER_255M__HR.html