Hi Tim, and welcome to the board, hope you find it very useful.
Most of the guys on here are from all around the world, so are a great source of not only gebneral info, but also desitination specific. There are also a few of us from the UK.
When it comes to 'best time to book', as I am sure you have noticed, there are no hard fast rules, we normally adopt the approach of when we see a price that we like, we grab it, then stop looking at prices for that cruise. Sometime ignorance can be bliss
The only thing I would warn about booking via a non-UK based/registered TA is that you may not be covered in the event of collapse of a company. I am guessing it is HIGHLY unlikely whe using a company that size of Carnival, but if you were to take up fly cruise you may find yourself on a flight that suddenly no longer exists, and ABTA/ATOl won't be available to you.
2 other notes as heads-ups.
If you book Princess again, watchout for their pricing of tours, they are fluid prices now, so the earlier you book potentially the cheaper the tour.
If you book a specific cabin on P&O, make sure you ask NOT to be on the upgrade list. Their policiy changed mid-2011 so that even if you booked a specific cabin, they have the right to move you to another cabin even if it is only one category higher, not one grade higher. We had a hell of a fight to move back to our chosen OG from the totally unsuitable OD cabin that 'they' considered to be an upgrade !! We still consider a Grade to be inside/outside/balcony/suite etc.
Hope you have many hours of fun on this site & look forward to hearing of your future plans.
Alan & Katrina