Being a SE phone owner (T28W, T39, T68i, and T68ie) myself, I try to keep an eye on the news on SE products and for firmware upgrades, I've read that some of the Car Phone Warehouse locations in London (such as the one at Chapel Street and John Lewis) do perform firwmware upgrades.
You might want to give
them a call and see which of their retail locations will perform firmware upgrades on the spot.
Regards,
David