From what I know, Brittania is a pewter alloy that’s designed to be harder and shinier. It’s different from regular pewter, which is softer and easier to form. I’ve worked with regular pewter, but not Brittania. I don’t think you can anneal pewter, but I might be wrong about that.
Here’s an old thread from Lisa Slovis who wrote a book about pewter working. It looks like her book is out of print, but you can track Lisa down from her website. I’ll bet she’d be happy to answer any questions that you have. She’s a really nice person.
Best of luck!
Jeff