This may be a bit disappointing for those of you who are looking for a way to repair it ourselves, so to cut the long story short - I've brought my phone to the Samsung service and they've fixed this in 10 minutes for free!
I did flash some custom firmware and kernels several times, had a yellow triangle and did quite a few other fancy things with my phone. But eventually the phone just died on its own, although being on almost stock firmware and kernel. Unlucky for me I was not in the country where I bought it and I didn't have any warranty documents at all. Few weeks later I've found this repair instruction - http://forum.xda-developers.com/show....php?t=1528256 and was able to revive the phone. Almost... if not counting that IMEI was reset to the default one, which is blocked by all UK carriers.
Finally, a few days ago I came to Russia, reset the yellow triangle, flashed the stock firmware, grabbed warranty documents and headed to the Samsung service center, preparing myself for a long wait and being afraid that they won't fix it until I have to go back to the UK, which would be very disappointing. I've seen stories that service centers can't fix IMEI and just replace the main board.
And I can't express how much I was excited when I was told they can simply re-set the IMEI and it'll take just 10 minutes! All completely for free!