Would stick with only one make of groupset, there is a love on BF for mixing and matching Shimano and Campag, but it real life, it's looks wrong, and can cause unnecessary issues.

Would avoid getting old components, as you are in the UK, except for ebay, there isn't the availability of parts over a year or so, that there appears to be in the US.

For Ultegra not matching Centaur, not following you there, Centaur is like 105, where Ultegra is the equivalent of Chorus, although if I was buying today, I would hold off till 6800 comes out.