Follow these steps and you should have the UK version of the Android Market up and running on your Galaxy Tab.
Be sure to backup all files as this would be a factory reset.

1. Open the Phone App

2. Enter the following info *#272*your device IMEI#
(your IMEI number is on the back of the galaxy tab)

3. Then choose XSG as the Sales Code option and
tap ok/install.

4. The tab will reboot and reset, this should take around
1-2 min. Then setup your tab again and you can see the Market app on the main screen and the menu.

Samsung Galaxy Tab - No Android Market In My Samsung Galaxy Tab