I have not been to the Canary Islands, but I agree with janfred, billybuzzy, and Cruise Fanatic on traveling to Europe. We've done several cruises in Europe, most recently out of Copenhagen last month. On that one, we had six countries and four currencies. Typically, the currency on the ship will be $US. In port, we used local currency (that we got in small bills before leaving home) and used credit cards for larger purchases. There are less expensive ways of using foreign currency, yet we didn't mind too much as we really didn't spend much.
We've also been through Heathrow several times, and had connections there last month. I agree with Cruise Fanatic that the Boston to London flights are pretty easy. It's not much longer than flying across the U.S. We have a favorite hotel in London that is located near Buckingham Palace, and I'm sure other members have their favorites as well. The transfer from London to Southampton is about 2 hours - I suggest using cruise line transfers for ease.
I say - go for it.