If I go into a pub today, and there are smokers present (evident by the smoky atmosphere), of course I can exercise my right as a non-smoker to leave, and find another pub more to my taste.

If a smoker enters a pub and sees non-smokers are present (evident by the absence of any smoke), he could exercise his right to leave, and seek out a smoke-filled pub. But how many smokers would do that?

It's true I can choose not to enter a pub if it already contains smokers, but supposing there are none present when I enter, and let's say I order a drink and a meal, and then a smoker comes in and lights up a cigarette and starts blowing smoke in my direction (whether intentionally or not). What am I supposed to do now? I've paid for the drink and the meal so I don't want to walk out - I wouldn't be able to get a refund either.

Yes, I could ask the smoker politely to stop, but that's likely to cause friction, and at worst a violent or aggressive response, on the grounds that it's his 'right' to smoke, regardless of the consequences to me or others in the pub.

That's why I'm in favour of the new law. It means I can be certain this scenario won't happen - at present it's a lottery for non-smokers, and the cards are stacked in smokers' favour, because non-smokers (all else being equal) don't affect them at all.