Email and ask if they accept bookings from abroad. That's what we did some years ago and have booked around 10 cruises that way now. You need to confirm that they will give the benefit of any price drops and also that they don't have a cancellation charge. You pay by credit card, but the payment goes direct to the cruiseline so there is no fear of the TA disappearing with your money. With the recent drop in the pound against the dollar, it may become less advantageous to book this way in the future (we just paid the balance on 2 cruises early so that we don't get any unpleasant surprises) Sorry I can't give recommendations - I'm sure it wouldn't be allowed on this site.
Go 2 pages forward & theres a similar question & answers.
Google cruises and start surfing?we just tried one site and it would cost us £3oo each MORE than we got it with a uk agent,maybe the last minute deals are better?