London is big yeah, but so well connected. The underground system from the O2 (for example) has you in Central London in under 20 minutes... It's doable. The opening ceremony could be in Kew Gardens or Hampton Court Palace. That would be stunning.I think London is far too big to stage a successful Eurovision event. It would be a fine tv show but the wider event for fans - and crucially local people - would be swallowed up.
If the UK did host, somewhere like Manchester, Liverpool or Glasgow would be far better because the whole city could join in the party and showcase themselves internationally.