score:5
Will I be able to use the flights connections route
Maybe, but I don't think you can rely on it. For starters, you need to check in with AC for your flight to Toronto. During Corona, check in for international flights is an unpredictable process. It often requires a manual doc check at the check-in counter. You may not be able to get a boarding pass online or airside at the gate or service counter, even if you can make your way there.
Will Air Serbia let me board at Belgrade
Unlikely, but you can always ask. Air Serbia sold you a ticket to London and as far as they are concerned you get off the plane in London and what you do there is none of their business. Some airlines will accept 3rd party "proof of onward travel" but it's safer for them if they don't and the terms and conditions typically spell this out.
Given that we are in Pandemic, I would NOT do this, especially not with a 1.5 hour layover (assuming JU380 + AC857).
Things are really unpredictable: Two weeks ago I flew from FRA to AMS. I physically HAD to show up at the counter to get a boarding pass and it took over an hours since everyone had to check in manually and they only had two counters staffed. A couple of days later I flew onward to Berlin. I had no trouble getting my boarding pass two days ahead online and it took me less than 5 minutes to get to the gate. You just don't know until you get there.