It probably has been addressed before, but so be it-
Advance fares are out, so you may as well book now.
If there is a rail strike with no service offered you will get a full refund, so don't worry.
Depending where your hotel is, you can either go from Victoria to Canterbury East (which takes around 90 minutes) or South Eastern High Speed (over HS1- the Eurostar line) from St Pancras to Canterbury West which takes just over the hour.
For the time saving on the actual ride, if you are staying near Victoria, it's not worth taking the tube to St Pancras, unless you want to!