My Wife and I will be joining the ship after you reach Venice on the 10th June.I would be surprised if the there are not a lot of Brits. onboard with the way that Princess have been marketing the Golden heavily here in the U.K.have a nice time and I am sure you will meet plenty of people your age.
Without wishing to put a downer on proceedings... I think the numbers of British teens will be dependent on whether they are taking GCSEs or AS/A Levels!! My two teenage sons will both be taking theirs from end May through June. Our own cruise in late July was dictated by the fact we wanted to be back home for the results!!! In fact I had to change my original date because it clashed with the A Level results.
Emily, there will undoubtedly be others your age on board (perhaps not as many as in previous years). My own children made many good friends last time. In fact one lad from California came to stay will us last year for 10 days and two years on many others still keep in touch by email.