The problem is of returning with a longer lens is that big lenses are a bummer to carry. If you are to pack a 500mm in your bag and the tripod to hold it on, that's it, that's your shot, there's simply no walkabout after. In other words, you are committing a slug of a journey and quite a few hours of time to get that single concept.
And the reason I am choosing such a high F stop is that lens compression makes the Eye and the Shard look close but they are not, hence, I want a significant depth of field so that the both appear in focus.
Finally, since I was out and about with this big set up, another shot that I had seen out and about on my travels was the Shard and St Paul's. I've had to crop here because a crane was blocking the view. You can see it peeking in in the bottom right hand corner.
I should say that the peaks of the Shard and St Paul's look so close because of lens compression, and not any Photoshop effects. This is straight from camera.