Police spotting (junction watch)

Since there's a no cycling symbol on certain bus lane such as the bus lane on Bloomsbury Way, those without such symbol would suggested that we're allowed to ride on.

I'm not sure what the distinction is, should cyclists only be on bus lane if there's a sign/symbol for it, or if there's a marking stating that cyclists shouldn't be on it?

You can generally assume that bus lanes are open to cyclists unless signage (bus only etc.) indicates that they're not. As far as I know, the only bus lanes not open to cyclists in London are contraflow bus lanes. There may be an exception somewhere, though. I doubt that there are many.