I've seen different limits over different lanes on the M25 occasionally. I wonder how that works if what the instructor said was correct?
Interesting, I thought I'd seen that too but it was stated very clearly that the same speed limit always applies to all lanes. Definitely not the case in other countries where you can even have different limits active in the same lane (e.g. for cars and trucks).
If the instructor was correct I guess you'd have to assume the display was faulty (as in the example we were shown where lane 3 had no limit displayed but 1 & 2 did) and take the lowest figure shown as being the current one.