Fletcher and fellow Paddock Motorsport McLaren co-owner Plowman finished third on the road behind Ian Loggie (RAM Racing Mercedes) and Shaun Balfe (Balfe Motorsport Audi), following a mid-race red flag during the pit cycle, but was promoted to first as both had their race one success seconds added to their race times.
TV actor Fletcher, who won the GT4 Pro-Am title with Plowman in 2019, had begun the second stint in seventh but was elevated positions when Alex Malykhin spun at Druids, Adam Balon had to serve a stop-go penalty for earlier contact and Morgan Tillbrook suffered an electrical glitch.
After pulling a crucial move on Mia Flewitt into the Hislops chicane, Fletcher then closed the gap to Loggie and Balfe, which meant he moved ahead when their respective seven and 10 second penalties for finishing second and first in race one were applied.
VIDEO: A dramatic British GT season-opener at Oulton Park
Having finished only 15th in race one after a track-limits penalty and a puncture, Fletcher was elated to score a maiden GT3 victory on his first weekend with a McLaren 720S, after switching from a Bentley Continental GT3 over the winter.
“I had little radio, my ears were playing up all weekend,” he told Autosport.
“It was funny, so it kept going in and out.
“I knew once I did the McLaren [Flewitt] I had some free air so I tried to push on as much as I could.
“I had no idea that we were on for a win, even on for a podium. I didn’t even know it was last lap as well.
“My head was in the game, I was really trying to focus and getting a couple of tenths in each lap.
“I was pretty lucky with traffic, I pushed really hard and I was really happy with that.”
Team-mate Plowman described the result as “a career highlight” after “coming into the weekend a bit on the back foot”.
“We made a few tyre pressure adjustments for the restart and Kelvin just drove brilliantly,” he told Autosport.
“He made moves when he needed to make moves and to close down the leaders at the end to get the jump was amazing.
“We definitely struggled this weekend – personally and as a team, we’re still learning this new tyre so we’re learning every time we go out.
“I think once we nail the set-up and know how to get this tyre in the window, then we’ll be a consistent contender.”
RAM protests result
The race result remains provisional for now after Loggie's RAM team lodged a protest.
Loggie and Jules Gounon were classified fifth following the application of the success seconds post-race, which Loggie called "bizarre".
“How can you have the same penalty for a race that only lasts 20 minutes?" he said.
"If a race runs 20 minutes and it’s a 60-minute race with a seven-second penalty, that should be pro-rata in my opinion. It should be 2.33 seconds.”
RAM boss Dan Shufflebottom said: “There’s nothing in the regulation unfortunately that applies to that situation, it’s obviously [something] we need to look at.
"Effectively we were given the penalty for a one-hour race but had an 18-minute race. And without a regulation that says that should happen, we don’t think it should have done.
"The regulations are not something that should be left to ambiguity, that’s how I felt it was.”
However, RAM's protest was rejected when stewards convened on Tuesday - but the team has now indicated it will appeal the matter to the National Court.
While RAM was left unhappy, other outfits had no problem with British GT’s interpretation of the rules.
Barwell Motorsport boss Mark Lemmer said: “That was the only thing you can do in those circumstances. It’s such a messy thing when the pit window opens just as a safety car with a weather choice, that was absolutely the correct procedure.”
Century Motorsport’s Nathan Freke agreed: “I have no issues. It was a tricky situation, no-one really knew what was going on. So I think red-flagging it and resetting it like they did probably was the only logical way of making it right.”
