Certainly a strange scenario and no mistake. Two forces with almost identical structure, doctrine, traditions and equipment fighting each other.
I'm inclined to think the advantage would be with Rhodesia, if only due to the UK's incredibly long supply tail and their lack of air transport capability back then. The Tanzanian-Zambian railway had not been built and I'm uncertain how co-operative the Portuguese would have been in allowing use of the Angola-Congo-Zambia line... bearing in mind part of it went through war-torn Katanga.
Wilson's government was not friendly with the RSA and while I doubt substantial RSA support for Rhodesia, I doubt they would be 'difficult' with regard to what passed over the border either by the same token.
There is considerable potential for the UK to suffer some embarassment at the very least, which I suspect was why HW ultimately decided to do nothing.