Avios miles are a *******

I have collected Avios purchase rewards by flying B.A. and purchasing petrol at BP for many years. Have tried on several occasions to use our miles for flights to UK and Mauritius and were never able to be accommodated. Our latest experience is for return flights to Johannesburg to Cape Town 10 May return 18 May 2018. With Avios we use 13000 miles each plus so called "taxes" of R1900.00 each on B.A. We have booked same days through Kalula at R1850.00 total each including fare and so called taxes each.