If you are filling your passport pages up, even a 48 page UK passport won't take so long to be filled up, meaning a trip to Hong Kong or the UK will be necessary.

Actually, you can get a new passport from the UK Visa Office in Beijing in order to accommodate your next L visa, to be issued by your spouse's hometown's PSB office. When you renew L's on the mainland, each L requires just 1 passport page - there aren't any entry/exit stamps, of course!