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Have you called X? If so then you would know that they have sold out of their allocation for flts from both LHR and LGW so it is cruise only and book your own flights - looks cheap an outside is only £634.06pp which for an 11n cruise is fantastic price.
There are plenty of flight options available from LHR & LGW but will all cost around the £735 mark which does make the deal less attractive unfortunately.
One other thought, cruises that start & finish in San Juan are often cheap & it maybe because of the fact that the locals, who naturally board in great numbers at their home port, at school holiday time their kids have the worst reputation for being completely out of control. I would strongly recommend you research the various cruise forums for some of the horror stories that have been written on this matter. San Juan as a cruise stop is quite an interesting port, but I would seriously reconsider using it as an embarkation point in view of what has been written in the past. Good Luck.. Wilba.
And don't forget if it is Quinceañera season ......hundreds of 15 yr old girls behaving as...well...15 yr old girls. These cruises out of SJU are magnets for these.