According to the Motorman report "the weather on Thursday's practice was intolerant. For the morning session the rain reluctantly held off but genuine English conditions prevailed later in the day." Thursday was the only practice day that year.
The during the GP "The rain began to fall and on lap six the two BRMs were 11 seconds in front of Palmer.."
"Geoghegan was experiencing trouble with his googles and he pitted for a visor without losing his position."
"With 30 laps gone the track was beginning to dry.."
Like you I didn't recall it being wet that day.