I live in London so here are my 2 cents..
New-build flats should be approached with caution (check another post I made today about the Portman Building Soc. halting lending to buy-to-let investors looking at new flats).
As for the areas. Canary Wharf is pretty much old news. There is always development down there but check the proximity of any property to Canary Wharf itself as this is where your prospective tennants will probably be working.
Greenwich is a desirable area to live in but it is quite a long way out.
East London generally is a pretty horrible place. With the exception of a few trendy hotspots (Victoria Park in particular). Though East London will see a lot of Olympic development.
Kings Cross - The area is indeed changing but it is still fairly run-down and sleazy, though it is close to both the city & the Westend which is a big plus. But this has been touted as the NEW area for the past 2/3 years and the prices of the developments around there seem to reflect that.
This year has been a bumper year for the financial district and it is expected that some of the vast amounts of bonus payments will make their way into the property market. You could be competing against some very wealthy buyers.
Apart from that rents have recovered somewhat but at lot of serious commentators still think that the UK property market is substantially over-valued. But the same can be said for Ireland, the US, Spain & half a dozen other countries.
If you want any further information just post a reply here. Try
www.foxtons.co.uk &
www.kfh.co.uk for two pan-London estate agents. Key in your budget and see what this gets you in various parts of London.