"For
Imperial Wizard membership you need 4 qualifying flights and another
150 points".
It's no wonder you're confused - it's one of the daftest anomalies on the BAEC home page. The "4 qualifying flights" bit never self-updates, but the "150 points" bit does
As a regular traveller to the same one of 3 or 4 business locations, I now always originate my tickets at the other end. I have yet to find a fare that isn't cheaper that way - sometimes dramatically so. To kick it off, you either book a one-way out, or a nice return ex-UK but with a really big gap between legs. I find a big gap between the legs does wonders for the fare.
Really though, I wonder if BA Silver is worth the effort. If I'm in T5 now, I go and sit in that wonderful cafe "Amato", underneath the South end Terraces. It's at least as nice as any lounge and much quieter.