I think the Amex 2-4-1 has the same fare rules as a normal award booking. They are shown below.
However, these don't show what happens after departure for changes (with cancellations after T-24 it's a "lose all your miles" game
. I suspect that you will lose all flexibility if you take the MAN-LHR flight and have a long stopover (I assume you're doing this to effectively extend the validity of the voucher?).
CHANGES TO THE DATE AND TIME OF FLIGHTS CAN ONLY BE MADE UP TO 24 HOURS BEFORE OUTBOUND DEPARTURE DATE AND ARE FREE OF CHARGE.
A CHANGE TO BOTH DATE AND TIME OF FLIGHT IN ONE CONTACT CONSTITUTES JUST ONE CHANGE.
CHANGES TO NAMES WITHIN THE BOOKING ARE NOT PERMITTED.
BOOKINGS WITH E-TICKET: CHANGES THAT REQUIRE TICKET RE-ISSUANCE CAN ONLY BE MADE UP TO 72 HOURS BEFORE DEPARTURE AND A CHARGE OF £30 WILL BE MADE FOR EACH CHANGE.
BOOKINGS WITH PAPER TICKET: CHANGES THAT REQUIRE TICKET RE-ISSUANCE CAN ONLY BE MADE UP TO 21 DAYS BEFORE DEPARTURE AND A CHARGE OF £30 WILL BE MADE FOR EACH CHANGE.
CANCELLATIONS TO ALL TICKETS CAN BE MADE UP TO 24 HOURS BEFORE OUTBOUND DEPARTURE FOR A FEE OF £30 PER PASSENGER. ALL MILEAGE WILL BE RE-CREDITED TO THE MEMBER'S ACCOUNT AND TAXES WILL BE REFUNDED.
MILEAGE QUOTES SHOWN ARE APPLICABLE TO THE CLASS AND ROUTE SELECTED.
ALL TAXES, FEES AND CHARGES TO BE PAID BY THE MEMBER (NOT BRITISH AIRWAYS).
AWARD BOOKINGS ARE SUBJECT TO AVAILABILITY AND CANNOT BE WAITLISTED.
IF FLIGHT BOOKINGS FOR AWARD TRAVEL ON BRITISH AIRWAYS ARE CANCELLED BY THE MEMBER WITHIN 24 HOURS OF DEPARTURE, MILEAGE IN RESPECT OF THE AWARD WILL NOT BE RE-CREDITED TO THE MEMBER.