I haven't tried but I think you could do it this way; three Amex points converts to one M&M point.

Amex to Emirates Skywards 1 for 1; Emirates Skywards to Heathrow Rewards 3 for 1; Heathrow Rewards to M&Ms 1 for 1.

M&M have just terminated their credit card in the UK, which means it's just about impossibe to earn M&M points unless you actually fly or stay in a hotel - very difficult in these times.