Download the latest 4.3 for UK - which would be - I9300XXUGMK6. Then follow instructions in the first post
You want to flash the ELL4 U.K. firmware from the stock firmware thread. Download the S2/S3 CSC changer app from the play store and set the CSC to BTU.
This will factory reset the device, so make sure you have your important files backed up.
Then proceed to flash MK6 4.3. Now everything will match.