Due to various bits of administrative faffing, I had to delay submission of my renewal forms. This means that it is getting rather uncomfortably close to the date of my visa's expiry. So I sought to make an in-person application. It's more expensive, but you get the visa that day and so it is guaranteed to happen before your current visa expires.
That is, assuming you can get an appointment.
I tried every office (Croydon/London, Sheffield, Birmingham, Glasgow, Liverpool, Cardiff) through the online booking system, repeatedly. None of them have appointments available. Please note also that some of these places are a good five-hour journey from both my place of residence and my work. I phoned UKBA, who informed me that every appointment at every office is booked through 31 December.
The really beautiful thing about this is that you are not allowed to renew your visa until within 5 weeks of its expiry date. Hence, by the time you're ready to submit your application, it is impossible to get an appointment.
The icing on the cake was finding out that since I filled out my form, it's changed. So I have to print out the whole 75 page application and fill it out again.