You can always purchase the Economy Select branded fare which will gives you increased flexibility (cancellation permitted subject to a penalty) that will allow you to purchase at a small premium over/above the non refundable Plus fare. My understanding is the Economy Select fare is only bookable via a travel agency (ie not sold on ba.com) which in itself could bring a raft of other restrictions.... but it is a solution to your problem.
This way you can secure the lower booking classes by continuing to book well ahead of departure and still earn Avios / tier points on your underlying cash ticket plus take advantage of paid upgrades (POUG'S and AUP's) as/when they happen nearer departure once you know your travel is going ahead. Accepting POUG's and AUP's invariably change the rules of your underlying ticket to non changeable non refundable so it's important you only pull the trigger on these once you know your trips definitely going ahead.
More information on the BA branded fares available along with what their general rules and restrictions and differences are can be found
here