Polesitter Rob Collard had led in the early part until he slid wide to allow Cammish to move clear for a 2s victory as Honda completed a clean sweep at the circuit.
Like in race two Cammish again made a sluggish getaway, which allowed third-starting Tom Oliphant to rise into second place as Collard remained unchallenged.
The FK8 Honda’s strong braking performance allowed Cammish to retake second place under braking for the Club chicane while Collard held a 0.5s lead.
Light rain arrived as Cammish twice aborted manoeuvres on Collard, again on the approach to the Club chicane.
But on lap 11 of 15 – the race shortened after a clutch failure for Rory Butcher led to an aborted start – Collard slid wide in his Vauxhall on the exit of the high-speed Church right-hander.
That killed his momentum and allowed Cammish to pass for the race lead on the run to Club.
The weather deteriorated further, particularly over the first sector, with Cammish avoiding all kerbs to maintain first place in the low-grip conditions.
But he was close to 2s clear at the flag, scoring his third BTCC victory in the process.
Collard brought home his Power Maxed Racing car in second ahead of a double podium for Team Dynamics as Matt Neal climb a place to third.
Following a clutch failure in race two, Adam Morgan’s hastily rebuilt Mercedes was a strong fourth ahead of a recovering Jason Plato.
The second PMR driver had climbed from 13th after his race one penalty, and ran without success ballast and profited from a scrap between Ashley Sutton, Josh Cook and Tom Ingram to climb the order.
Race two victor Cook was sixth ahead of Oliphant, as Ingram eventually prevailed in his race-long duel with Sutton.
Andrew Jordan chipped away further at team-mate Colin Turkington’s points lead with 10th, while race one winner Tordoff was 11th.
Turkington finished 13th, behind Jake Hill in the Audi S3.