Hi Mark!
Thank you for visiting the Moms Panel!
Unfortunately, the PhotoPass+ only lasts for a period of 14 days, from the first day of use, and cannot be extended. I had exactly the same question as you, because as we are also from the UK, we usually stay at Walt Disney World for a longer period than two weeks. You are very welcome to use the regular
PhotoPass during the other seven days - perhaps use that during the first week, and then activate the PhotoPass+ for the second and third weeks. I am sorry there is no other way around this, but the new PhotoPass+ is such a great value that I still think it is worth using, even if not for your entire stay.
If you have any other questions, please come back and ask me - I would be happy to help you plan your magical vacation!