I found out today (after not much digging - it obviously hadn't annoyed me that much!) that you can change it in about:config (type about:config into the address bar, and accept the warning). Just change the key "browser.search.defaultenginename" to "Google UK" - or whatever you please. I found what to change it to by copying the value of the "browser.search.selectedEngine" key.
Now I don't have to replace ".com" with ".co.uk" every time I do a search in this way, and it automatically returns me pages from the UK.