Interesting point about Avios. I have points both through qualifying BA flights, and from Air Miles accumulated over many years of credit card use.
Last year, I needed a return from Newcastle to Hamburg, which obviously needed connecting via Heathrow. Interestingly my searches for flights on the BA site offered different results to those found searching the Avios site. However, neither offered worthwhile connections for the outward leg. The only flights offered were the last flight out of Newcastle to connect to a LHR-HAM flight the next day for my chosen dates. Out of interest, I decided to search for a number of different dates, but all returned the same result. Clearly useless as an overnight stay in London would be necessary. Was this deliberate policy I wondered, for when I searched for flights for cash payment, I was offered innumerable options, and as the prices were reasonable I opted for a one way option to Hamburg on the BA site.
Interestingly, the return offered lots of options one way HAM-LHR-NCL using Avios, so I cashed in some of my Avios points for a free return.
Call me cynical, but I feel the system was manipulated to make it difficult to use my accumulated Avios points.
On the plus side, the previous year I managed to get free return flights using Avios between NCL-MUC and NCL-CPH.
I have yet to test the situation this year.