Thanks for all the comments people! They are very appreciated.
OK, as some have noted London is a pain to work in as a photographer. Being a professional photographer and having to carry a big bag and tripod I'm normally spotted quite quickly. But of course, it all depends on how you handle the situation.
Here's some of how I dealt with certain scenes:
Tower Bridge/ Gherkin etc in front of City Hall: believe or not this is private property controlled by More London. When I was asked "are you a pro?" I just looked at the guy and said "no mate, I'm just up here for the weekend and like London." Security aren't always the brightest bulbs when it comes to photographers and so off he went and I carried on
St Pancras: never ever had a problem here. I know Network Rail don't like us photographers but the only issue I ever had was when I tried photographing the front of the Eurostar through the glass as there is a security guard who sits there and took issue. I might try again though.
Liverpool St: This one was an issue as it's done looking through a glass screen on private property. Like a lot of London it's one of those places you think is public but it isn't. On this occasion I had my tripod set up and the camera pressed up against the glass to avoid any reflection. A security guard came over and asked what I was doing (always a mute point for me). He said his boss had asked me to move on. I just said to him I've nearly finished and just keep talking to me. I gave him so much speel and yack that I was able to finish. When I was finished and packed up another guy came over and had a right go at me. Just ignored him!
King's Cross: I just went and looked around. Security didn't see or say anything. Best thing for places like this is to use an interval of 1 or 2 seconds. That means you get nice smooth footage very quickly and if you are found chances are that the short interval has given you enough footage. Stock libraries tend to take a minimum of 5 seconds of footage. So 2sec interval x 120 frames (24fps) = 4 minutes.
Natural History Museum: No one said anything as I used my Manfrotto table top tripod. The best 25€ I have spent on a piece of kit. It has paid for itself many many times over. It helps you get things when tripods are banned as lot of people look at it as just a support and not a pro tripod.
The other views that a lot of you won't know where done as I was given the relevant permission; paid for a hotel that had a view (between £250 to £400 per night) or paid for a view.