The message
from Chris Hodges contains these words:

Postie sees 2nd class stamp, treats as 2nd class despite other stamps.
Which is annoying because a 1st + a 2nd isn't much more than the cost of
1st for 60-100g and beats queueing in the PO, but often gets treated as 2nd.

What I've found makes the biggest difference is printing the postcode
instead of hand writing it. Makes 2nd class letters arrive like 1st
class.

--
Skipweasel
Pay no attention to that man behind the curtain.