For several years, you could only apply 3 months before you wished to enter the UK. However, about a year or two ago they started allowing you to select a date up to 6 months in advance. So you should be fine.
Having said that, we usually advise selecting the EARLIEST possible date you wish to enter the UK... because if the visa is processed by then, you can move on that date (the visa will normally start about 7 days before the date you put), but if the visa isn't processed by then, it will simply be made valid starting a few days after the visa decision is made.
The only thing to consider with using Jan 1st 2021 as the intended travel date is the fact that if the visa is issued earlier than that, it still won't start until late Dec 2020 or early Jan 2021, which means you will not be allowed to move to the UK before that date.
So that means that say, for example, your visa is issued more quickly than you expect and you get it in September or October, you will still have to wait until January (the visa start date) before you can enter the UK. Whereas if you put a start date of say, September 1st, and you get the visa in October, you can move to the UK immediately.