Is it possible to unlock my Samsung Galaxy 3(GT-i5801)?
You should call your carrier in the UK and explain that you need to unlock your phone because you are travelling.  Unless they're being particularly unhelpful a carrier will normally give you the unlock code for that purpose.

Edit: It looks like the unlock code may be stored on the device and accessible as with many Galaxy phones, most easily if you are rooted.  Check out this AndroidForums guide.