We're not UK but even in the US it's financial cheaper to spend 10 days because of flight prices for us. That keeps us from flying on the weekends either way when prices tend to be at least $100/person higher.With the cost of flights from the U.K., very few people would consider coming for just a week. The destination for many is Florida, not just WDW.
And the cost of a 14 day ticket is better value per day than a 7 day ticket; in fact often we can get 14 days for the price of 7 days. But that’s obviously without G+ added.
@graham, it seems they are extending the validity of the 7 day ticket to the full 14 days again. So if you’re staying offsite and will be doing other stuff too, you might be able to get the 7 day ticket and spread your visits out.