Regardless of galvanic compatibility, if titanium ever strips, snaps, or gets stuck, you'll have a hell of a time drilling it out. I generally avoid it for that reason.
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Found these class 12.9 bolts on Amazon UK, but they're not zinc plated.
-Found a place called
Celtic Industrial Fasteners that isn't too far from Bridgend, and they carry 10.9 bolts, but it all appears to be coarse thread (1.75). Perhaps they could special order the correct thread.
Everywhere else I've looked so far either doesn't carry it, or doesn't have an online storefront. Might prove easier to smelt some ore and forge them by hand.
I normally buy such hardware from McMaster-Carr, but... I don't think they ship to the UK currently.