RDPX is London based and mentioned a good new sushi place off clerkenwell green, but you'd be needing reservations.
That is Sushi Tetsu - its booked up 2 months in advance and is not cheap but you can email them in case of cancellations. We have managed to get a reservation for my birthday in May but it took 2 hours of redialling and even then only got table for 6:00-8:30! I shall report back but am expecting great things.
If you really want sushi then Pham Sushi is very good and not too expensive - it is on Whitecross Street in EC1. Between Barbican, Old Street and Moorgate tube stations.
http://www.phamsushi.co.uk/ I was there a couple of weeks ago and it was very busy, which is always good sign in sushi place.
Central restaurants the burgers mentioned above are all right on trend at the moment it's true... Polpo has a few incarnations and is also very good. Another one we like a lot is "Bocca di Lupo" in Soho.
http://boccadilupo.com/. If you sit at the end of the bar here you are right at the pass, and everyting that goes out of the kitchen makes you think "oooh that looks nice".
You might even want to see if you can get a table at The Ivy - if you keep the wine to the house it is actually pretty reasonable. The food is vert good though not michelin style, but the staff are very good and the room is as good as it gets.
I don't know if you like exhibitions, but there is a fantastic Chuck Close show on at White Cube on Bermondsey Street at the moment. It is free (commercial gallery) and huge show, and the gallery itself is an amazing building. About 40 yards up the road is "José" a Seville type tapas bar which is tiny and buzzing and truly excellent. Both are about 5-10 mins from London Bridge - I went to both on Sunday and it was a fine day out.
[If you go there on Saturday, it is also v close to Borough Market, and if you get to the market at about 4:40 you will get lots of stall holders knocking excellent food out at 50%. Well worth a trip and a pint in the Market Porter afterwards.]
One other thing that is just fantastic is to go to the bar at the top of the Centrepoint tower by Tottenham Court Road tube. It's best if you call ahead and make reservation, but just go up and have a cockatail, or even just a beer. The cocktails are good and the view is AMAZING.
http://www.paramount.uk.net/
Feel free to PM me if you want any more info on anything.
Robert