I don't think IGMP Proxy should be enabled, you need IGMP snooping which I believe is achieved (as you have done) by enabling IPTV for the connected port.
Port forwarding is not required. I assume you are using the 192.168.2.xxx subnet on the TP Link rather than 192.168.1.xxx
The standard network is using 192.168.0.xx
Iptv network (with just the bt box attached to port 1 of the router) is 192.168.2.xx
Did you give it that address or did the TP Link allocate it by DHCP? I would have thought the box would need to be on the 192.168.0.xx subnet.