proof of sponsor/fiance's identity in visa application

My girlfriend is going back to apply for the fiance visa soon, and doesn't want to take my current passport with her for the application (she's going for a month, and is worried I might need it in an emergency).
Any other documents she could take?  Perhaps my old expired passpoort?

a certified copy of the sponsor's passport is acceptable (any solicitor can certify a copy.)

Is acceptable and required.  If you didn't have a passport then a birth certificate would do.  Also, someone shoot me if I am wrong, but ALL pages of the passport need to be copied and certified, even the blank ones.

No, Kit.  The portrait & id page is needed only. 

The idea that blank pages need to be certified for sponsors is an internet myth. 

The myth is rooted in people who apply for ILR or citizenship, where residency needs to be established.  This has nothing to do with sponsors or even entry clearance applications.  But it persists nonetheless.

What about my old expired passport?  Are there no other documents that could be accepted (e.g. photocard drivers license)?

Have you (or she) looked at the application form or guidance that goes with it?  It will give the list of documents that they can accept...

You need to establish [1] your identity; and [2] your capacity for sponsorship.  The single document that does both is the passport.  A driving permit only accomplishes [1] identity.

Capacity for sponsorship will be covered by payslips/proof of income, so looks like I just need proof of identity right?  So drivers license should be ok?
We don't really want to pay £50 to a solicitor for a photocopy, if we can avoid it.

no, I think you misunderstand garry- you need to prove that you are a permanent resident of the UK (eg, a British citizen) which shows that you are eligible to sponsor your fiance's visa application. Only a passport (original or certified copy, not expired) or a long form UK birth certificate will suffice for that purpose.