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When the question is "which flight engine lets me filter via (unusual request)", I've found the answer to almost always be ITA Matrix. Using it, you can do searches requiring a certain transit point using their advanced routing codes.
Specifically, adding X:MAN
to a search would require a transfer in Manchester.
Your results would then be only flights that have stops in Manchester. You can then book the results from ITA Matrix using a tool like BookWithMatrix
I think Hipmunk used to also have a similar feature, but they unfortunately seem to have gotten rid of it. Best of luck!