Some green manures are easy - grasses, buckwheat, and phacelia will fit anywhere in your rotation.   Use them where you like, when you like!

The Legumes fit in reasonably easily - they bring a resource to your rotation cycle and fit in especially well before nitrogen-greedy crops like the brassicas.   For example, a spring sowing of Tares would be a good preparation for your winter brassica; dig it in just before you transplant the young plants.

The Brassica family green manures are much trickier - and I do NOT know the answers.   Some ideas - I say clearly I have not tried them; I don't intend to encourage the club-root in some of my beds.

• Sowing Mustard after a crop of potatoes, and prior to a planting of winter brassicas.
• Growing Fodder Radish as a winter green manure, allowing it to rot down, and dig in prior to spring and summer brassicas.
• Using them ONLY where you never grow brassica crops!   Mustard in the greenhouse?   Radish in a permanent runner bean bed?   Radish as a first crop in a new allotment, in beds which will not get brassica crops for at least a couple of years?