It's a Colt .45, but in his hand, you aren't going to see if it's a 1911 or 1911A-1, good grief!
Churchill insisted that his newly formed Commando regiments have .45 autos as their primary sidearm, and many paratroopers, SOE, SAS and other British troops carried .45 autos in WW II. Even in WWI, a lot of .45 M-1911's got used by both British and Canadian officers, who bought their own sidearms until 1920. Churchill bought his in 1915, and it IS a .45 ACP, not the .455 auto variant. Man at Arms did a story on his pistols some years ago, with excellent photos, allowed by the present Lord Churchill. I think his guns had to go to museums after the terrible 1997 gun law passed.
I have seen a photo of the general that Connery played, but the flap on his holster was shut, so we don't know which handgun he carried. Maybe the British producers asked Gen. Urquhart or paratroopers who knew him?
But that looks like a .38 cartridge pouch above the holster. ??
Photos from the real Arnhem battle depict paratroopers carrying both a revolver and an auto pistol. The distance was such that you can't be sure if the auto was a Browning 9mm or a .45 Colt. Could easily be either.
In addition to the large number of .45's bought from Colt, we sent a lot of Lend-Lease .45's to the UK. I used to own one.
Movie studios have had problems with .45 ACP blanks. So, the gun in some scenes may be a 9mm or .38 Super Colt, but one shot of the muzzle looks as if it was a .45 at least some of the time, and it's intended to be a .45.
Star and Llama 9mm's have also subbed for Colts on TV. I think I read that in scenes where Stacy Keach actually fired Betsy, he used a Star 9mm which worked better with blanks. ("Mike Hammer")
If you look carefully at the photo of Lord Lovat in his return from the Dieppe raid, you can see the butt of his pistol well enough to see that he had a .45 auto, the most common handgun used by such special ops troops. His rifle was a personally owned Mauser-actioned sporter. I've read that he also used a Winchester M-70 and a Mannlicher-Schoenauer carbine. In, "The Longest Day", Peter Lawford played Lovat with a Mannlicher on D-Day. He's shown leading his regiment ashore behind a bagpiper.
Finally, about the app. 10,000 .455 Eley caliber Colt autos bought in WW I, they went mainly to the RAF after the war, and in 1941 or '42, RAF Coastal Command had all transferred there, keeping all the guns and their obsolescent ammo in one command. Because Coastal Command rescued downed German aircrew from the Channel, they would face armed Germans and they probably wanted effective handguns.
I think it's unlikely that Maj. Gen. Urquhart had a .455 Colt. It was almost surely a normal .45 ACP. In addition to Colt pistols, Commando units used Thompson SMG's even after Stens became available. They used the same .45 ammo as the pistols. Paratroopers may have also had some Thompson then. But the only SMG photos I've seen from Arnhem showed Sten MK V's. I am not at all sure if Browning 9mm pistols reached troops by the time of the Arnhem battle. Of course, an individual paratrooper may have captured a Browning from a German. They used FN 9mm's heavily, especially among SS forces and paratroopers.
Oh: yes, Connery fired the Colt in the movie. He shot a German who saw him through the window of a Dutch house.
"A Bridge Too Far" was a superb war film, very close to the truth. My only complaint is that the planes that bombed the Germans were the wrong sort. I guess that real Typhoon or Tempest fighters simply weren't available to the producers.
For an American account of Arnhem by a paratrooper who fought there, I suggest, "The Road Past Arnhem." The author, Donald R. Burgett, wrote several books about his days with the 101st Airborne in WWII. As for his guns, he favored the Garand rifle and carried his own nickeled .45 auto and a captured P-38 9mm. He liked the M-1 because it worked well and a solid hit on a German usually meant a dead German. The .30 Carbine was weak, and the Thompson too heavy and too short-ranged.
BTW, Maj. Gen. James Gavin, commanding the 82nd Airborne Div. also favored a Garand rifle, unusually so for a general officer. Besides it and a .45, he wore a Randall Model 1 knife.