I thought Kenya was now on the list of countries for which two departures/arrivals in Europe were permitted, so long as one is a transit without stopover. From memory, others include: Tanzania, Uganda, Nigeria and Ghana.

So: DUB-LON-AMM // CAI-x/LHR-NBO-x/LHR-Asia would be fine.

(also, as I'm writing this, it also seems like the XONEX could be used for LON-AMM as well, as I only count two stopovers in Europe so far...LON [the first time] and AMM)

However, if the trip begain in the UK, as in LON-AMM // CAI-x/LHR-NBO-x/LHR-Asia, another problem would arise: while the second transit in Europe is still fine, there is no provision for a second international departure from the country of origin. Therefore, in order to avoid a forced NBO/JNB overland trip, a non-UK origin is required.