GETTING THE APPOINTMENT:
When your application and documents are ready you can either send your application by post (fee 395) or you can call home office (any of the offices, Corydon, Birmingham Glasgow, Liverpool) and get an appointment (fee 595). You must remember that even you are applying by post, after 4 weeks you will be asked to come with your partner to attend the interview. In my opinion, it depends on your budget, if you can afford, directly take an appointment. Although it will be expensive but you will finish everything on one day and same date can be useful for your next application for ILR (Indefinite leave to Remain, after two years). Use home office website to get the right phone number for the appointment.
In my experience, most of the time, various appointments days will be available, you need to check your availability and discus with the operator. Operator will provide you a reference number and a date and time. When you are reaching to home office make sure that you are at least 30 minutes early for any security checks.
As you finish your security checks and fee payments, you will be asked to wait for your number and sit in a hall. When your casework will call your number, you and partner will sit together and provide all documents to the casework. In the meantime case worker will ask you various related questions, about you and your partner’s early relations (if any of you have), where you are studying and working etc. One thing I must warn that on various forums, it was written that caseworkers have taken separate interviews from both partners. There is a possibility that postal applications can get separate interviews but with my personal experience there was only one meeting with caseworker behind the window, like your student visa. If you have gone to Croydon for your student visa renewal, it will be the same experience, apart that this time your partner will be with you.
If you provided all required documents and your caseworker is happy that everything is genuine, within an hour you will get your visa. Good luck for your next application for ILR and your relationship with your partner.