Bilaterals would only apply on direct or non-stop flights between GB and ZW.
Emirates already have a service to HRE but it would be the subject of an agreement between the UAE and the thugs that currently misrule Zimbabwe. No agreement would be needed to carry traffic from GB to ZW via an intermediate routing point.
In respect to your question, the BA traffic rights might still exist nominally, and until more recently Air Zimbuggered operated LGW - HRE. I would think you would need to contact the UK CAA and the CAAZ (00 263 4 585073 / 87). I also think you are probably wasting your time but it would be interesting to see the outcome.
I would guess there must be considerable demand for flights between the two countries due to the large number of Rhodesians/Zimbabeans who have been forced to leave their home country. However looking at today's BA and SA flights LHR - JNB, there are only about 20 passengers connecting to HRE/BUQ/VFA, and about 50 tomorrow.