Our tours brochure arrived too late (and from Chicago ???) for us to book in advance, so we did so on the ship. But I am pretty certain that you can book your tours on the Cunard website. I thought it was under the 'cruise personalizer' section, but of course as we don't have a future booking :-( can no longer access that part.
If you can't find anywhere on the site, then maybe drop a line to Cunard, or check with your travel agent.
Hope that was of some help,
Alan & Katrina
Last: Oceania Nautica Aug 2016
Next: Cruise Maritime Magellan Nov 2016
Then: P&O Oceana December 2016
Then: NCL Jade August 2017
Golden, Grand(2), Caribbean, Royal, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(3), Oriana(12), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(7), QE, Caronia(2),
Oceania Nautica, Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey