Personally, I would go for an international chip as then it should be readable anywhere in the UK. My pets both have US-style chips (one HomeAgain, one 24PetWatch), and the folks at Heathrow (and my vet's office, incidentally) have scanners able to read them. However, there's no guarantee that every shelter/animal control office will have them, so if your pet does go missing the chip may be useless.
The only alternative I can think of is to buy the 15 digit chip @pettravelstore.com and the cheap scanner and just use that. It would be more expensive than the AvidEuro by about $60 but would mean he had the 15 digit chip that works all over the EU now. No idea if the cheap scanner is any good either.
You shouldn't need to get a scanner, just the EU-style chip.
I am also still unsure if I should get the titer done after the rabies vaccine. We will definitely not be moving before January - and I think the Defra site says we won't need it after Jan 1 - but I guess I won't feel confident about that until someone gets to the UK and reports back that it was ok.
Yes, the titre should be optional as of January 1. The checklist on DEFRA's site for the new requirements basically want you to: 1) do the chip first 2) then do the rabies vaccine 3) then wait 21+ days (the site still lists 21 days, apparently other documentation for vets in the UK lists 3 months) 4) get the EC 998 form signed off by a USDA state vet 5) travel. You can leave the titre information off the EC 998 form.
Worst case scenario, if you wanted to do the titre anyway, it's not that expensive. I think I paid about $120 for my dog and cat to get done.
Hope that helps.