I have just installed a velopsystem not without it's challenges. It is all working now but I have the same issue you describe due to the different networks.
Did using the same subnet on the BT router and velop router solve the problem?
You may have to put the Velops into Bridge Mode for the config to work. I have a Home Hub with my Velop Parent connect by ethernet to the Hub. In another room I have the Child unit setup in bridge mode with the Parent. Connected to the child module is my Apple TV. This all works fine. I do sent two networks 1) the BT home Hub and 2) the Velops... regardless of the network I connect to I get access to all my wireless devices. If you have a second Apple TV that will just connect to the wireless network created by the Hub or Velops networks, the same goes for any other wireless device.
You set the Velops to bridge mode via the app on Android or iOS *( which is how mine is set up).
hope this helps!
I appreciate that. I have already looked into that but then I cannot use the advanced features of the velop like device priorities or parental controts etc
yes, that is one of the disadvantages of bridge mode. But if it allows to the network you want it may be a price worth paying.