I would imagine a good quality photocopy would suffice. She's a British citizen and normally, they would accept a photocopy of the husband's passport when he's a british citizen, after all, if she's british then she doesn't have to apply for a visa to enter the UK so it's not her they will be passing judgement on. I'm sure that by using the passport number, it would be a mere formality for them to check it out.
Iain.