The OP and the replies are based on wanting to move up to 'Jets'. i.e that is your aim. A plane is a plane is a plane but you want the jets so you have pretty much answered your own question.
However I'd make sure that this African airline is quite stable because if it isn't, then you'll just have the TP experience like you would have had in the UK.
I'm on the 744 and for me, it is crap. Moved onto it too early 25 years old!. I'm sure I'd love it when I'm 50 years old and content with pushing buttons but at my age, I miss flying. I wish I never left my 747-200F job because that was classic flying at its best - 5kts of wind and you'd probably have more confidence in flying it by hand to FL100 rather than trust the autopilot! (slight exaggeration granted).
285 hrs? Congratulations mate. If I was in your position, I'd take the African Job. Because as you said, you'll fly the Dash-8. So worse case is you don't get the A330/340 but you move back to England with Dash-8 hours (same as staying in UK). Or best case is you get the Airbus.
When I started out, I really wanted to stay in England.But was given the chance of a 737-200 in the Philippines or a TP job in the UK. I took the jet job even though there was no progression to a larger a/c as they didn't fly them! I just wanted the jets at the time.