Most places in London accept Euros too..but wouldn't hurt to have pounds just in case.
I am not sure how good an idea it would be to have the ship do a currency exchange ( pounds to Euros) or if they do that?
travelers checks may be hard to use in port ( many places request your passport for ID when trying to use them, which is usually held thru-out the cruise by the ship.. so makes it difficult. Easier to just credit card eveything.
Would suggest if you want to use them that you make copies of your passport to carry with you ( infact do it anyway -good to have).