you do need to book an appointment

JJ, British citizens are required to apply from the CVASC branches in UK, according to what I know. There are three branches in London, Manchester and Edinburgh. I checked all the websites of these three branches. They have different requirements on the appointment. If you apply from the branch in London, you are required to make an appointment in advance. If you apply from the branches in Manchester and Edinburgh, appointment isn't a must though the branches suggest you should make an appointment in advance.

Regarding of the applications made by mailing, it's true that almost all Chinese embassies don't accept mail applications. However, these CVASC branches accept mail applications. Of course, it takes long time (about 10 days) and you need to pay extra for the service.