No. There is no such thing as a direct flight from UK to SJU, all will go via a USA gateway.
I flew direct to St Juan when taking a Princess cruise in the Caribbean, but this was a charter flight booked through Princess. Kind regard.
Continental have a very reasonably priced flight from New York to San Juan. We flew from Belfast last year with Continental and then on to SJU. If you can, take a couple of days in Puerto Rico before your cruise - it's surprisingly good.