No, joining the EMU would not make better conditions for long-term investors in Britain, because the pound is more stable than the Euro. Look at the mess in Greece. There is no reason that Britain would want to be part of that. Germany is strong economically, and all they have had to do is bail out Greece.
The United Kingdom has been right to avoid joining the Economic and Monetary Union for as long as it has. Investing in the United Kingdom is now, and has consistently been, stable. The EMU on the other hand has been subject to countless issues, causing economic instability for member states. If the UK opted to join the EMU, current investments in the United Kingdom would significantly depreciate, and the United Kingdom's appeal to new investors would only diminish.