To attend appointment after the specified date

I’ve applied online for my first extension of Family Route - FLR(M) application and paid for the Super Priority service.

In the ‘Further actions’ step on the online application portal, I was told to ‘Attend an appointment to provide your documents and biometrics by 13 December 2019’.

I would like to know if I could attend the appointment on or after 16 December 2019 with the understanding that this will be subject to availability. The reason for that is my current visa is expiring on the 11 January 2020 and I would like to apply within 28 days before it expires so I would be able to apply for ILR (SET(O)) later on without any shortfall.

1. You don't qualify for ilr on form SET O if you hold a spouse visa on FLR M.

3. If you do not attend your biometric appointment within the specified timeframe, your application will be invalid..

3. You should have only applied nearer the time of your current visa expiry.

What is your UK immigration history?