You can do this by editing the search.ini file inside your Opera profile folder. In Opera go to Help, About Opera, to find out where your profile folder is. (Mine is C:\Program Files\Opera\profile.)
Close Opera and copy and paste the search.ini file to your desktop as a back up. Open the original search.ini file in Notepad. Under "Search Engine 1" change the reference from google.com to google.co.uk and click on File and then Save.
The line should read URL=click here
(There is a search.ini file directly in the Opera Program Files Folder i.e. not in the profile folder. This is not the one you want to edit.)