First, you need to unlock the bootloader referred to [TUTORIAL] Sony Ericsson Xperia Arc bootloader unlocking by lollylost100.
Then, download the system image you needed:
[New 2011-05-27] R800i_3.0.1.A.0.145_UK_system.zip (155.9 MB)
PS: It's highly recommand to flash the unrooted .145 UK tft ([ROM] R800i_3.0.1.A.0.145 - Generic UK Firmware by thomassafca) via flashtool and flash this rooted images becuase the Baseband version was updated.
Downlaod the Android SDK (android-sdk_r11-windows.zip) via http://developer.android.com/sdk/
Extract android-sdk_r11-windows.zip to C:\
Extract the system.zip file to C:\android-sdk-windows\tools and downloaded the SYSTEMFILE.img file. (Filename will get different)
Open "SDK Manager.exe" from C:\android-sdk-windows
Press "Cancel" from the "Choose Packages to Install" box.
Go to "Available packages" and double click "Android Repositoey".
Choose "Android SDK Platform-tools, revision 4" and click "Install Selected".
Click "Yes" to restart the ADB.
Click "Close" and close the SDK Manager.
Go to "C:\android-sdk-windows\platform-tools" and copy AdbWinApi.dll AdbWinUsbApi.dll to "C:\android-sdk-windows\tools"
Make sure that the .dll files in C:\android-sdk-windows\tools
Unplug the USB cable and power off the phone
Press the "Search" button and plug the USB cable.
After that you will see the BLUE Notification light.
Go to the Start menu and click Run. Type cmd and press enter.
cd \android-sdk-windows\tools fastboot devices
If you got XXXXXXXXXXXXXX fastboot that means your phone successfully connected.
Then, you can flash the system img to phone.
fastboot flash system SYSTEMFILE.img
After Flash operation complete, reboot the phone!