In that case, my post above stands, that is central london. In terms of safety, if you are on your own, then earlier evening might actually be better inasmuch as there is safety in numbers, more people around. Apply the same safety as you would anywhere really, London is no better or worse then any other city centre with perhaps a slightly lower ratio of drunks to sober people around. Be observant. Keep kit to a minimum and make sure its somehow fastened to you to prevent the opportunist grabbing and running. You will of course need a tripod so you can get those lovely long exposures.
edit: to add... be cautious around the London eye, its technically private property, and the security chaps can be very officious, but to be honest, some of the best views are further away from it anyway. Also, if you actuallt venture into docklands [as opposed to viewing it across the river or from the outside], be prepared for security to accost you there too, although a call ahead might just clear that for you, they do provide permits
