Hi @Anonymous User
Based on the information you have posted, you won't be able to buy and save the 6 months at £20 per month sports offer and activate it towards the end of December when your Virgin deal runs out.
Because you need to purchase the offer no later than the 4th of December 2019.
If you are eligible for the BF sports pass offer where it should be available in a BF link at the top of the Passes & Vouchers screen page, then if you decide to purchase the offer the sports pass will automatically start and you won't be able to save it for a later date like a NowTV day or week pass.
Should you buy this BF offer, then during December you will have two ways of accessing Sky Sports (either through Virgin or NowTV) though you would have wasted the first of six months on your NowTV sports pass deal.
If you don't mind outlying a full payment upfront, then possibly look at the NowTV sticks with the bundled one month sports pass which are selling for £20 per stick at places like Currys & Argos etc on a Black Friday Sale.
Should you go down the stick route, then don't start or activate any of the sticks until you are ready to start watching Sky Sports via NowTV once your Virgin deal is up.
