Hi D,
The best bet is first of all to make sure that the BT box does in fact output an RF channel. By this I mean, make sure that you can connect it's 'RF out' connection to 'Aerial In' on a near by TV. In analogue mode, scan for channels, and make sure the box is sending a picture. I have a feeling that it may only have what is called 'RF loop through', in this case, it uses the signal that the aerial supplies, and passes it on to your TV set, but does not generate a signal onto the aerial cable. In this case you would need to buy an RF modulator to sit between the scart output of the box and the input to the aerial amp. Let me know if this is necessary, and I can suggest some options.
If the first test works OK (the box does give a signal through the aerial out connection), plug the wire that you used to have connected to RF out 2 of the Sky box into RF out on the BT box. Link the box to the main TV with scart or HDMI as available. Having done this, you may need to retune the other TVs, if the BT's RF output is on a different channel than the Sky box was.
After this, get a cold drink and watch some TV
I hope this helps, let me know how you get on.
Cheers,
Colin
Edited to say, don't forget that your old Sky system can be left in place if wanted for the free channels.
This link suggests that the older BT vision boxes did have a RF modulator fitted but newer ones don't.
http://www.radioandtelly.co.uk/btvisionbox.html
This being the case, you may need something like this to get things working - no connection with the company, etc.
http://www.tvcables.co.uk/cgi-bin/tvcables/uhf-modulator.html