You'll probably get more responses if you'd posted in the General Discussion forum but maybe one of the mods can move the thread? The actual mix depends on the country as well as the time of year and can be anything from 10/90 up to 50/50 Butane/Propane. I noticed the same, I got 280 miles from a tank in my Saab running over to Minsk compared with 240-260 in the UK. However, some of that can be down to the continuous constant speed cruising you can do in Europe which you can't do over here due to the amount of traffic and congestion.
To fill from a Butane bottle would be the same as filling from a red Propane bottle in as much as you'd need to invert the bottle so you get liquid and not vapour and use a pump. Not impossible but not straightforward, see http://www.diy-lpg.co.uk/articles/files ... ed-up.html
for Simon's article on his pump. You also need to bear in mind that BBQ gas has no road fuel duty added to the price so HMRC would get offended if they found out. It also depends on how much you would have to pay for the BBQ gas bottles. A quick Google suggests you'd be looking at around £28 for a 13kg bottle. 13kg is roughly 26 litres so you are paying over £1 a litre so rather than cost savings, you'll be paying more compared with the 60 odd pence a litre buying LPG at a filling station.