There must be something in this Doctor Who/Blake's 7 connection for the currently showing series. First we had the guys running around in the B7 helmets -- I think that was Varos -- then we had Servalan in The Two Doctors, now it's Avon's turn.
I'm sure a bit of research would either prove or disprove my theory that B7 had just ended and DW got some spare actors, more budget and some left over props in return. I seem to recall B7 starting (here) in 1980, when I was in grade 6. I don't think it went more than 5 series. Although, wasn't there a break before the final series? I really must do some research.
Actually, another theory - was B7 at the height of its popularity and they got the actors across to give DW a bit of added cachet? And some hand-me-down props.