Sonic X will definitely air in the UK on the 28th January.
It will be on every Wednesday on CiTV (childrens ITV/Carlton) at 3:40pm.
Source: Radio Times Online
Curiously though, the first episode to be aired is Infiltrate Area 99, not Enter the Supersonic Hedgehog, which most know is the first episode in the series. Not sure as to what the reason is, but I am trying to find out.
Edit: Someone has reported seeing an advertisement for Sonic X to air on CiTV on Wednesday, January 21 at 3:40pm in the UK. So this upcoming Wednesday may be the true starting air date.