If mobile data for a particular SIM is disabled then it will not connect whatever the roaming settings for that SIM are (I have roaming enabled). That is the optimum solution I can see as setting a SIM as the data sim does not stop the other one from using data, as described in the thread I linked to.
It appears from NextG's post that O2 can do that for me, but using a code as AIS do would be so much more convenient and gives the assurance that a change in the settings doesn't.