Hi Paul and welcome,
1. I'm unsure if you are supposed to do this, but you can read's your dad's BT Terms & Conditions. That said a wireless signal does extend beyond the house boundary as you have found out.
2. It depends upon how much you want to spend on the wireless link. Wireless works best on a direct line-of-site so the best solution would be a point to point link in the eaves, or on the chimneys, or from facing windows if possible, perhaps using the 5.8 GHz frequency band to avoid problems in your dad's house.
Tell us a bit more of your local geography and your budget and no doubt several will put forward solutions.
Kind reagrds,
Walter