Although HDMI bypasses all the color-encoding issues of NTSC vs. PAL, there are still format differences in terms of the number of lines and the frame rate. For example, the format corresponding to a regular (standard definition) American TV signal is 480i/60, while that for a regular British TV signal is 576i/50. (These are the normal 525 and 625-line broadcast systems, but as several lines in each frame are blank or used for other purposes, 480 and 576 are the number of lines which carry actual picture information.) There are similar differences when it comes to high-definition resolutions, such as the 720p/60 and 1080i/60 used for HD digital broadcasts in the U.S.
Quite a number of HDMI-equipped TV's on the British market will accept the American formats, but you would need to check the specifications to be certain. Of course, depending upon the software and hardware combination you are using on the computer, you may also be able to configure the HDMI output for a British/European format.