I don't think it'll happen for a long time, if ever. I think the UK will remain a monetary outlier, like Switzerland or Norway, and will remain at the mercy of international financial events. I agree with William Buiter who says that there is no reasonable argument for a small, highly open economy like Britain to retain monetary independence, and that the belief that an independent national monetary policy allows you greater greater scope for effective macroeconomic stabilisation is an example of the monetary fine-tuning fallacy.
http://blogs.ft.com/maverecon/2008/05/when-will-the-uk-wake-up-and-join-the-euro-area/ "To have a large, internationally active banking sector and financial system, your currency has to be a serious global reserve currency if you are to be able to provide the lender of last resort and market maker of last resort services required to minimize the risk of a bank run or market liquidity crunch bringing down large chunks of your banking system. You can decide to take the risk of running a large globally active financial sector with a local currency like sterling or the Icelandic krona, but you will be taking an unnecessary and costly risk. Sooner or later that risk will be reflected in your cost of capital and make you uncompetitive.
So, if the UK wants to remain the seat of the world's financial capital, there is only one choice: adopt the euro now, and wonder why you did not do so in 1999."
He says, interestingly (and ominously):
"It is not much of an exaggeration to describe the UK as a hedge fund, a highly leveraged entity, borrowing shorter than it lends and invests. It has a lot of short-maturity foreign-currency-denominated foreign liabilities and quite a lot of illiquid, non-sterling denominated foreign assets."
We all know (now) what happens to hedge funds when the music stops.