You can apply in person at the Chinese consulates in London, Manchester or Edinburgh, but if you do not live near either of these cities, the cost of getting to the consulate can be more expensive than paying a third party to collect it for you. The consulates do not accept postal applications.
I was a bit apprehensive about sending my passport and payment to an unknown firm found on the web, but the whole process was completed quickly and efficiently - I used www.visatochina.co.uk/China/ the cost was around �40 which covered everything and I got my passport back within a week. Suggest you send the passport by recorded delivery and include a recorded delivery envelope for it's return.
Have a great trip, it's an amazing country.