Upvote:2
The ‘standard’ processing time is 3 weeks, so in theory you could wait until as late as 10 January 2022 to apply, providing you can get an immediate appointment at the visa application centre.
You might be able to use the priority service (decision within 5 days) if it’s available at the centre where you’ll be applying.
However, a decision within these timescales is not guaranteed, even if you pay for priority. So in practice, the answer is to apply within the 3 month period prior to your intended travel date as soon as you have all the required supporting documents.
Source: https://www.gov.uk/guidance/visa-decision-waiting-times-applications-outside-the-uk