If I look at an O class one way BRU-LHR-MAN it seems the change fees are:
Brussels to Heathrow (London)
Changes
Time/date changes permitted at any time before each flight departure for a change fee of €80 or an upgrade fee of €80 plus any difference in fare. All sectors may be repriced. Changes subject to availability. Fees apply per ticket
If you want to cancel your flight
There are no refunds except for any government & airport taxes
Heathrow (London) to Manchester
Changes
Time/date changes permitted at any time before each flight departure for a change fee of €80 or an upgrade fee of €80 plus any difference in fare. All sectors may be repriced. Changes subject to availability. Fees apply per ticket
If you want to cancel your flight
There are no refunds except for any government & airport taxes
As noted by
orbitmic, no plus fare or basic fare is offered, just an economy fare. It seems €80 is likely also the change fee for the OP under the fare booked.