DaveHam Loc: Reading UK

This is not a sensible question. There is no magic answer.

What are you planning on shooting? No one can say 'take this lens' and expect you to cover all your needs unless you are shooting in a specific style. What would you take to photograph your own location? Take that. London does not require a specific lens. You may have to walk around a bit to get the composition but that's what you have to do whatever lens you take.

sb Loc: Florida's East Coast

It sounds like the OP has the kit lens that comes with the APS-C cameras. So if that is the case, he might want to consider adding on a wide-angle 10-18mm lens ( great lens for about $300), but if he really does not want to make this a "photography trip" he will probably do pretty well with what he's got and not worry about lugging around too much gear.

EdU239 Loc: The Northeast

I think the 18-135 would be an excellent choice. You might want to add the EF-S 24mm pancake, or the EF 40mm f2.8 for slightly more reach. They are inexpensive, small, light, easy to carry and in my view great for urban travel. If it's in your budget, I'd second jerryc41's recommendation to consider taking a good compact with a decent zoom for the city.

Incidentally, while you're in London, I would strongly recommend taking a Thames commuter or sightseeing boat out to Greenwich or Woolwich. Aside from the observatory and various neat museums in Greenwich, the views of the city and bridges, particularly at night, are wonderful. Have a great trip.

bweber Loc: Newton, MA

I agree that with a crop sensor camera the 18 - 135 is ok, but you may want a wider lens. I have made many trips to Britain. and I find that shooting in London and many other English cities and towns a wide angle lens is important. I used 15 - 30 on my full frame camera a great deal of the time.

Roger Lee

The only problem with the pancake lenses 24 or 40 is that they are f2.8. Other than that they are good lenses, the nifty 50 shines because of it's 1.8 aperture. And it shines, maybe a little less than the 50f1.2, but still good.

camerapapi Loc: Miami, Fl.

I am a Nikon man but I am sure I can give you advise based on my own experience. The 18-135 lens you own will satisfy many of your needs of your trip to London. I would only add a wide angle zoom but understand that wide angles need a closer look at your subject. Nobody buys a wide angle to "cover" a wide view.
I am sure Canon has excellent wide angle zooms. Tokina also offers good ones for the cropped and full frame sensors.

David in Dallas Loc: Dallas, Texas, USA

One can do things with modern equipment that was not easy or practical several decades ago (before the digital revolution). For wide-angle needs it is practical now to take a series of overlapping shots and make a panorama from them. (It is important to keep the front of the lens in the same spot as the camera body is rotated, to avoid parallax problems with nearby items.) Some cameras provide a panorama feature built-in, but I use a product called Autostitch®, which is very easy to use. With the pixel count available in modern cameras it is also possible to simulate a longer lens by simply cropping the picture to a smaller size and enlarging it.

I'm a Nikon user and my walk-around is the 18-200mm VR2 zoom. I think the OP will do well with the lens he has. If his tour is a guided one, there will likely be little time to be swapping lenses as he travels--staying with just the one lens will be much easier. (And also reduces the likelihood of getting dust, etc. on the sensor.)

speters Loc: Grangeville/Idaho

I would take my 16-35, my 70-200, my 135, my 400 and several others, also I would take one of my MF film cameras with a bunch of lenses as well (I take the Pentax 645nII, leave the Fuji at home - its just too big).

Peter W Loc: Kent, England

Lens sounds good but add a wider one. With the poor state of sterling buy a Sigma 10-20 mm while you are over here!

Klsing Loc: Georgia

These are awesome. I would love to do Scotland and Ireland also but maybe next trip.

Klsing Loc: Georgia

I have the 70D so a crop sensor. Thank you for your suggestions and I think I will look into a wider angle lens