Apple have not made PowerBook batteries in a very long time so any true Apple-branded ones you find will have now been on the shelf too long.
I just checked my records: the last PBG4 battery I ordered directly from Apple was in Dec 2009 and, within a few months, they has stopped selling them. That makes any surviving stock at least six years old, and lithium-ion batteries sit unused on the shelf rather poorly.
There is a US vendor I trust that ships to the UK and has new, house-brand batteries for PowerBooks. Caveat: import fees and VAT may make their price point uncomfortable:
http://eshop.macsales.com/item/NewerTech/BAP15AL54RS/
OWC are an established vendor with an excellent reputation for technical expertise and customer service. However, unless they have a physical UK presence, returns may be a bit dicey. You can certain e-mail them for before-the-sale support and inquire about how they handle returns should the new battery prove unsatisfactory.