I don't think that London would allow a independence referendum to take place in Scotland if it would be perfectly clear that the pro-independence campaign would easily win. For example, look at how the Spanish government refused to allow Catalonia to hold a referendum, where the majority of people according to opinion polls is for independence.
But nobody said that the pro-independence campaign is going to win easily. There is all to play for at the moment with the polls still against independence, but shifting towards the pro-independence camp in the last month or so. The big issue will be voter turnout - those against independence are far more likely to stay at home rather than actually get out and vote. The pro independence camp on the other hand has a very strong 30% that will get out and vote come rain or shine.
The key has always been to convince another 10-20% of the merits and by my experience (not necessarily shown in polls) we are succeeding on that front. People are more open minded about independence now than they have been in years. Everything is just ready for the big offensives of 2014 in the run-up to the vote.
There are also very important cultural differences at work here; Spain was a Fascist/Autocratic Nationalist state just 40 years ago, thus ideas like simply refusing referenda or blocking democracy isn't considered such a terrible thing. The British government knows however that if they tried to block our referendum they would be seen as interfering, causing popular opinion to shift firmly against them. We would probably win the referendum if they did something perfidious like that. HMG know they should step out of this and let it take its course, providing support to the indigenous Westminster parties in Scotland. That is exactly what they are doing.
Part of me actually wants Westminster to try something like blocking us. It would make life a lot easier for campaigners.