you can only flash your phone with the same variant FW, equal or greater than the one you have.
If you have the singapore SW12, you can upgrade to singapore FW 48, but don't for the UK retail FW 48
then, you just need to follow the instructions from that page, the same i will post again:
Firmware EF81
Prepare your mobile
* Please, remove your SIM-Card from the mobile
* Switch your EF81 on
Check the currently installed firmware
* type in *#06#, now the IMEI number is shown in the display
* switch one step right with navi-key to see the complete "Configuration"-information
* please note the information written below the following parameters (you will see further information by scrolling down with the navi-key):
Script name: EF81_de-BRDHandel-std_27_0123_001 ( You need only the part of the name that is printed in bold letters.)
* Your current SW-Version you'll find under SVN:
Download
Select the firmware you need, according to the information you read behind the parameters "Product:" and "Script name:"
* Select the correct value in the listbox "Product"
* Select the correct name in the listbox "Script name"
* Press the download button
Update
Connect your mobile to the PC using one of the following cables compatible with your EF81:
* Data Cable DCA-140 / Sync Station DSC-100
* start the downloaded firmware file (.exe) and follow the instructions
Important hint: If you update your EF81 mobile phone with this firmware it may happen that all received SMS, MMS, Pictures, Sounds and other data, stored in your inbox, are lost!
Please save this data, examply on a memory-card or a PC.
and welcome to sr-f