can i use an apple extreme and express purchased in the usa to set up a wifi network in the uk?

Yes, assuming that you have the right kind of AC plug adapter, but this would not be recommended because the U.S. and U.K. use different WiFi channels, so there may be some issues if you try this, and you may likely see some messages about a region WiFi channel conflict.

The bottom line here is that you won't know how well things will work until you try them out.

For example, the U.K. uses Channels 1-13 for the 2.4 GHz band. The U.S. uses 1-11. Some of the higher 5 GHz channels may not be legal in the U.K. so it would be a good idea to set the AirPorts to use channels 30-48 on the 5 GHz band.

You did not understand what I said. The Extreme allows 1-11. The U.K. allows 1-13. You won't be able to use 12 or 13 on the AirPort Extreme that you have.