This is all to do with the "Faster Payments" system - now that the technology is there it is no longer a long winded task for the banks to first check the money is there before transferring it - at an instant it is available....
http://www.apacs.org.uk/payments_industry/faster_payments_service.html
Faster Payments Service
Faster Payments is the first new payments service to be introduced in the UK for more than 20 years. For the very first time phone, internet and standing order payments can move within a few hours - almost at the touch of a button.
The Faster Payments Service enables electronic payments, typically made via the internet or phone, to be processed in hours rather than days. To date 13 banks and building societies are committed to the new service. In the future, other financial institutions will be able to join, either as members, or to access the system through agency arrangements with a member - just as they do with other payment systems.
The new Faster Payments infrastructure was launched on 27th May 2008. From this date, the founding member banks will be gradually implementing their own plans to deliver the new service to their customers. A phased approach to rollout is being adopted to help ensure that the service is launched smoothly, securely and with total reliability for customers. The new service will run alongside existing payment schemes in the UK such as Bacs and CHAPS.
The development of this new service is a massive and complex project involving significant investment. It will revolutionise the UK's automated payments infrastructure and will be comparable to the best in the world.