Sorry to hear that.
1. Usual problem is phone not recognised by computer and getting the drivers from Kies can be a problem so download and install from here:
2. Now in sammobile the most recent UK/Ireland firmware is:GT-I9000 - United Kingdom / Ireland
Regions: Austria, Belgium, France, Germany, Hungary, Italy, Spain, United Kingdom
Download to computer and unzip to get a single file
3. Start Odin and select PDA then open the unzipped file to get a TAR file in the PDA box.
4. Put phone in download mode and connect with USB and MAKE SURE it is recognised by Odin with a yellow box under ID:COM
5.You're ready to start.
If it doesn't go through the flash and hangs for more than 10 minutes you are going to have to flash the PIT file as well, but you need a different source for that file, so come back.
PS If you can't get into download mode it can be forced using a USB jig like this: