I would be a bit more charitable than some of the people on this thread. In many cases school treat "trial lessons" as jollies in the aeroplanes. "Look, there's my house" sort of trips, rather than serious flying lessons. Providing a briefing isn't perhaps the done thing for a trial lesson. When I had a trial lesson, the instructor sort of assumed I could fly, and got on with some paperwork while I flew!
This was quite good actually, even though it sounds bad, as it meant I was definitely doing the flying!
I would also suggest that the instructor who gave you the option to fly 1) was under a bit of time pressure, 2) didn't really want to fly that day, but gave you the option in case you'd be disappointed otherwise.
I would suggest that you arrange another time, and make it clear to them that it's not just a jolly, that you want to take it seriously, perhaps because it might lead on to other things. Say you'd like a briefing beforehand. Try and arrange it for a nice day! Good luck with that last bit! Actually when I did my trial lesson it was a bit grey and miserable with a 1600ft cloudbase - I know because I chose to bob along just below the clouds! Tremendous fun! It was either the best or the worst thing I ever did...not entirely sure which. I flew in there a few years ago and said to the guy on reception, "Do you know how much money you've cost me over the years?!"