The grass strips that you talk about could be part of a number of different schemes, all having different management requirements.
If the ‘2 metres’ that you talk about is next to a hedge or ditch then it is possible that it’s a protection zone under cross-compliance (GAEC 14). If that is the case then it is permissible to top this grass margin anytime, although it is strongly recommended that it be done outside the main bird-breeding season, which is 1 March to 31 July inclusive.
If the grass margins are ELS buffer strips (2, 4 or 6 metres) then you are allowed to cut them in the first year of your agreement. Cutting the margins in the first year is encouraged, as it will aid establishment as the grass tillers and out competes most broadleaved weeds. After the first year, 2 and 4 metre buffers can only be cut once again without a derogation. With the 6 metre buffer strips the 3 metres closest to the crop needs to be cut annually after mid-July, while the remaining 3 metres needs to be left (managed as you would the 2 or 4 metre buffers).
It is possible that some of the wider strips you have seen are set-aside headlands, which can be cut again at anytime, although you should avoid cutting between 1 March and 15 July when you may disturb or destroy nesting birds and prevent flowers from seeding.