I use those surplus MOD trousers, though not on the bicycle.
I sometimes use them on the motorbike (a pair rolled up in my panniers for me to put on if I get caught out when wearing summer kit), and more often use them when working outside. One problem with them is that the legs are quite baggy (albeit with zips at the bottom to nip them up a bit) to allow you to put them on whilst wearing boots. That would equate to very flappy on a bicycle, and possible chain/chainring interactions unless you do something at the ankle.
They can also have big holes in them where you'd normally expect a waterproof pocket. to allow you to put your hands into your normal trouser pockets. Worn with a big jacket that covers that they are fine, though many cycling jackets are quite short.
Good value though, if the limitations work for you.