Are these UK based companies you are dealing with?
I would be very surprised if it was not upgradeable via firmware although I found
this thread that shows the Japanese version as only supporting 2 languages.
WIth regards to lenses I guess it is what you are going to do with the camera, the sigma is a good lens but the canon F2.8 is better in build and optically. I own the non IS F4L and wish I had the IS version from time to time especially when the light is fading however it is a cracking lens and my favorite.
I have used the Canon F2.8 non IS MK1 and one thing I will say is it is very heavy so as a walk about lens you will know you are carrying it so if you are not shooting in low light much if it was me then I would lean towards the F4L IS.
If you do need the low light handling and you are going to shoot handheld then you are going to need to keep your shutter speed to match the focal length taking the 1.6 crop factor in to account to ensure sharp images if you are shooting handheld which will probably mean upping the ISO if you what to keep the ISO as low as possible then I would go for the Sigma or save a bit longer and get the F2.8 IS.
The one thing I did not mention over F4 to F2.8 is the extra bit of blurring (DOF) you will get for isolating subjects which is another consideration.