When does an LPA become 'active' — straight away or only when I lose capacity?

It depends what you put in section 7 of the Property and Financial Affairs LPA. There are two options. "As soon as my LPA has been registered" means your attorneys can act for you immediately after registration, even while you still have full capacity — useful if you want them to help with finances while you travel, for example. "Only when I don't have mental capacity to make decisions" means they can only act once you've lost capacity to make the decision in question. The Health and Welfare LPA is different — it can only ever be used when you lack capacity. Almost everyone gets this wrong, so think about which one fits your situation.