We have usually found it considerably cheaper to book with a US agent, with savings of up to 50% on UK prices. Even when adding in a flight it was still much cheaper. We normally book our own flights, anyway, as we combine a cruise with a stay in our home in the US, so only ever look at 'cruise only' prices.
For example, last September my daughter and I cruised on Voyager of the Seas, booking with a US agent. There were no flights left if you booked through a UK agent, so we would have had to book our own flights anyway, so the comparison wth flights was exactly the same as if we booked with a US agent - i.e. no matter who you booked with, the flight had to be independently sourced.
We paid just over $700 dollars each - the exchange rate at the time was around 1.62 dollars to the pound, making a total price of around £432. UK cruise agents were quoting around £700 for the same period. We often find the price is the same in dollars as it is in pounds. Naturally we booked with the US agent!
We also researched a cruise on Allure in late September this year with the British company, Virgin Holidays. They wanted nearly £900 EACH for flights which we could source ourselves for around £500! They also wanted hundreds of pounds for an overnight stay in a hotel - which we could have sourced ourselves for a fraction of that price. Their price for cruise only was also around 50% more than that quoted by American agents.
Of course there may well be times when it is cheaper to book with a UK agent - we found a competitive, and comparitive price last November and ended up booking with a UK agent, but more often than not it IS cheaper for us to book through an American agent - IF we are allowed to, and if we fulfil the criteria.
I am sure you can be lucky and find something cheaper from the UK, but how many Americans actually book through a UK agent? I wonder if Americans have such restrictions on who they are allowed to book with. Do you know, Tony? Would you be able to book with a UK site?
But surely anything which reduces the amount of choice of agents you can book with can only work in the cruise lines favour? For them to squeeze out the cruise agents who passed on some of their commission to passengers means that there will be less choice for the customer, and ultimately this will lead to us all paying more for our cruises. I can't see why the cruise lines should be able to dictate how much (if any) of the agent's commission can be passed on to the guests - if they are willing to accept a smaller commission in order to stay in business and offer good deals to their customers, then why should it bother the cruise lines - unless it is to force higher prices by limiting choice. Bit of a monopoly situation maybe?