(edited to emove long but unreadable image) Found the full rules (I think) for an A flight on ITA Matrix. Here's a severely pruned extract from them. Again they seen to be internally contradictive in terms of refund pricing and non refundability. The other limitations are detailed and numerous, especially in terms of flights that cannot be used.. I can't understand how there is so much about other flights, stopovers and further variations when the document specifies the only valid flights. An inefficient boilerplate generator? That might explain some of the variations but could introduce some legal uncertainties as well.
Apologies for the previous image, the capture and prune seems a bit 'off''. So here are some bits as text.

BA Flight Rules extractFIRST CLASS EXCURSION FARES A
APPLIES FOR ROUND TRIP FARES
FOR ADULT
TICKET DESIGNATOR - APP
...
THE FARE COMPONENT MUST INCLUDE TRAVEL BETWEEN LON AND SIN
ON
ONE OR MORE OF THE FOLLOWING
BA FLIGHTS 0011 THROUGH 0012....
Category 6: Minimum stay requirements TRAVEL FROM LAST INTERNATIONAL STOPOVER MUST COMMENCE NO
EARLIER THAN 5 DAYS AFTER DEPARTURE OF THE FIRST
INTERNATIONAL SECTOR.

Category 7: Maximum stay requirements

TRAVEL FROM LAST STOPOVER MUST COMMENCE NO LATER THAN 12
MONTHS AFTER DEPARTURE FROM FARE ORIGIN.

Category 8: Stopover restrictions

ORIGINATING UNITED KINGDOM -
UNLIMITED STOPOVERS PERMITTED IN EACH DIRECTION
LIMITED TO 2 FREE AND UNLIMITED AT GBP 75.00 EACH
...
CANCELLATIONS
BEFORE DEPARTURE
CHARGE GBP 150.00.
NOTE -
BEFORE DEPARTURE OF JOURNEY CHARGES APPLY PER
FARE COMPONENT TO ALL PASSENGER TYPES. INFANTS
WITHOUT A SEAT ARE NOT CHARGED THE FEE.
----------------------------------------------
WAIVED FOR DEATH OF A PASSENGER AND PASSENGERS
TRAVELLING COMPANIONS.
----------------------------------------------
IN CASE OF ANY NO SHOW WITHIN THE FARE COMPONENT
THIS FARE COMPONENT BECOMES NON REFUNDABLE.
-----------------------------------------------
REFUND PERMITTED WITHIN TICKET VALIDITY.
-------------------------------------------------
...
AFTER DEPARTURE
TICKET IS NON-REFUNDABLE.
NOTE -
AFTER DEPATURE OF JOURNEY FARE COMPONENT IS NON-
REFUNDABLE.
...
CHANGES
BEFORE DEPARTURE
CHARGE GBP 300.00 FOR REISSUE/REVALIDATION.
NOTE -
WITH THE EXCEPTION OF TICKETS CHANGED IN THE US
ANY PENALTY FEE MUST BE COLLECTED VIA AN EMD.
REFER TO SPEEDBIRDCLUB.COM OR BATRAVELTRADE.COM
AND YOUR GDS FOR INSTRUCTIONS.
--------------------------------------------
CHARGE APPLIES PER TRANSACTION - PER PERSON FOR
ALL PASSENGER TYPES.
INFANTS WITHOUT A SEAT - NO CHARGE.
--------------------------------------------------
A CHANGE IS A DATE/FLIGHT/ROUTING/BOOKING CODE
CHANGE. NEW RESERVATION AND REISSUE/
REVALIDATION MUST BE MADE ON THE SAME DAY.
...

Category 33: Voluntary cancellations

REFUND MUST BE REQUESTED BEFORE DEPARTURE OF JOURNEY.
REFUND REQUEST REQUIRED BEFORE ORIGINALLY SCHEDULED
FLIGHT OF FIRST UNUSED TICKET COUPON.
CHARGE GBP 150.00 PER FARE COMPONENT. IF ALL PENALTIES IN
PRICING UNIT ARE PER FARE COMPONENT COLLECT EACH. IF MIX
OF PER FARE COMPONENT AND PER PRICING UNIT CALCULATE EACH
AS SPECIFIED AND COLLECT HIGHEST. NO CHARGE FOR INFANT
WITHOUT SEAT.
...