Can't find the prohibition in BA's CofC. This is as close as they get
c. Using flight coupons in the right sequence and changes in itinerary
1. Your ticket is valid only for the transportation shown on it from the place of departure through any agreed stopping places to the final place of destination. The fare you have paid is based on our tariff for the transportation shown on your ticket. We will not honour your ticket and it will no longer be valid if you do not use all the coupons in the sequence provided in the ticket.
Seems like a loophole if you use two separate tickets.
You are correct that other airlines are much more definite about it. Northwest for example say
C) USE OF COUPONS FROM TWO OR MORE TICKETS ISSUED AT ROUND TRIP FARES FOR THE PURPOSE OF CIRCUMVENTING APPLICABLE TARIFF RULES (SUCH AS ADVANCE PURCHASE/MINIMUM STAY REQUIREMENT) IS NOT PERMITTED. NW AGENTS AND AUTHORIZED TRAVEL AGENTS ARE PROHIBITED FROM ISSUING TICKETS, COMMONLY REFERRED TO AS "BACK TO BACK TICKETING", WITH THE INTENT TO ABUSE AND/OR MISUSE RESTRICTED ROUND TRIP FARES. AGENTS FOUND ISSUING SUCH TICKETS MAY BE LIABLE FOR THE DIFFERENCE BETWEEN THE FARE PAID AND THE FARE FOR TRANSPORTATION USED. IN ADDITION, IN EXERCISE OF ITS REASONABLE DISCRETION, NW MAY, TAKE SUCH ACTION IN RELATION TO AN OFFENDING PASSENGER’S WORLDPERKS ACCOUNT, INCLUDING TERMINATION OF MEMBERSHIP AND FORFEITURE OF ACCRUED MILES.
NW HAS THE RIGHT TO DENY TRANSPORTATION TO PASSENGERS FOUND UTILIZING TICKETS IN THIS MANNER UNLESS THE DIFFERENCE BETWEEN THE FARE PAID AND THE FARE FOR TRANSPORTATION USED IS COLLECTED.
(Sorry about the caps - blame NWA.)