There have been a few issues with a number of BA Heathrow - Africa routes, this may have been one of them.
Taag i'm preety sure have shown interest in the London route, as there is no competition directly from Luanda really anyway.
However, 2 problems.
They are waiting for more 777's before they can work out expanding to new destinations (they lack aircraft on the ones they serve at the mo. - so starting new ones is not on their immeadiate agenda!)
Heathrow is virtually impossible to get a slot into, and so, Gatwick airport remain the only real possible option; with Luton and Stansted showing little interest to them (i assume you know why.)
The problem with Gatwick is that if Air Namibia in any way feel under pressure from Taag, taking away potential for customers into Gatwick via windhoek (Air Namibia serve gatwick as well), they may drop the temproary suspended codeshare partnership they have on the Luanda - Windhoek route.
However, dispite this, it is a strong considerable option for Taag.