Hi, I'm in the U.S., and not sure how well this number will work, but our website lists this phone number for the UK:
United Kingdom: 0044-8448008340, 09:00-17:00 Mon-Fri
http://support.asus.com/Hotlines.aspx?SLanguage=enThey may not know which stores will have spare AC adapters in stock, but you may want to try to ask for the numbers of a few stores.
The G73 also uses a larger adapter than most of our other notebooks though, so may be a little tougher to find, but should still be possible for stores to order, so hopefully somebody has a spare in stock.
Underneath your AC adapter and notebook (under the battery?) you should see the rating label of 19V, 6.32A if you are using the 120w AC adapter and the same 19V, but higher Amp rating (around 7.9A?) if you have the 150w AC adapter (might be more difficult to find?) and want to get the same one.
Good luck!