Interestingly, it seems that they have changed 8.3 again to bring it back into line with the previous situation, which would be the remaining membership year and all of the subsequent year.
This would be a nice reversion if it is real:

8.3. On receipt of a satisfactory application form, British Airways shall at its discretion issue to the nominee a discretionary Silver/Gold partner card which will be valid for a minimum of 12 months from the date of issue. For nominees who are existing British Airways Club members, the discretionary British Airways Silver/Gold card period shall begin from the date of acceptance by British Airways of the nominee and last until the nominee’s membership year end date and for the 12-month period thereafter. For nominees who aren't existing members of The British Airways Club at the time of nomination, Silver/Gold cards will be valid for a period of 12 months from the date of acceptance by British Airways of the nominee.