I think you best option may be to consider a UK departure NO flying more days at sea but choice will be resticted and fewer ships do the canaries in the summer. For August a quick look only found a few trip heading south to med from UK Also this is prime school holidays(you have kids or are teachers?) so budget will be important and family cabins tend to sellout.
You will get a better selection if you include the Med and even more if you add baltics/fjords
Tranasatlantics are next but one long flight. At the top of the heap is QM2 to new york and back.
Also consider the style of ship and think about budget because it will have a significant impact on choice.
Watch prices closely this year to give you an idea of what late booking options there might be the same ships tend to do same similar itineries each year so try and find a trip for this year as practice for next.
Papers, Teletext and cruise web sites all have info. just got to dig it out
Get on the mailing list for some of the agents it is one of the best ways to catch deals.