Tbh I'd say the Halfords stuff is fine. I'm working in one of their stores over the summer whilst I'm back from uni so I've mixed myself up a couple of cans of Titanium silver which I've only used to spray my gearknob insert so far but the match is, from what I can tell, pretty much perfect.
When you go in there ask them to mix a 300ml of Titanium silver (paint code 647). If they can't find it on their paint system just tell them (very smugly) that the only paint they need to use is 'H730'. But make sure they shake it well first as it'll come out way too dark if it's been sitting there for a while.
HTH