You can search for a flight first and buy the points if it looks ok, then repeat the search immediately and buy the fare as the points credit at once.
You can specify the airline and/or the departure time to get different results with AV. But in the end it's often easier to buy just the long leg on AV and pay a Y fare for the short one.
There's not many awards to MAN and you have to risk a connection in BRU or FRA. But AC864 YUL-LHR is available in C on 12.6, and UA958 ORD-LHR in F on 11.6. If you really want MAN the best bet is to start from an unlikely LH port like DFW or ATL, for example 11.6 DFW-FRA-MAN is available in C if you ask AV for LH flights