I know I haven't lived in the UK since 2002, but Test Matches always used to start on a Thursday. Since when have they been scheduled to start on a Wednesday?
In the period where Test cricket was in the doldrums, they wanted to take every chance to maximise ticket sales. Day 1 of a Test, plus the Saturday and Sunday (if no rest day) where the easiest to shift. If they started late in the week, there was limited chance off an early finish meaning dishing out loads of costly refunds on those Sunday tickets.
Nowadays with every day of an Ashes Test sold out regardless, they can start when they like.