The movie shows the British soldiers holding the L1A1 Self Loading Rifle (SLR) with black plastic butt stock and fore grip. These were not trailed by the British Army until 1974 and were then phased in over the next 8 years. The rifles should all have had wooden butt stocks and fore grips.
The issue of Melody Maker seen while Brigid chats to Gary Hook in the bedroom is fake, Bowie never got a full front page until he became a commercial success in 1972. In fact apart from his hit in 1969 (Space Oddity) he would not be much of a talking point in 1971. He also had long hair in 1971 so it was an old photo.
When they're receiving their weapons from the armoury, the store clerk doesn't perform the safety check - cock, hook and look - to show them that the weapons are safe and unloaded. Similarly, the soldiers shown also have carried out their own safety checks too but didn't.
When the soldiers are woken to be told their overseas deployment has changed, the corporal who comes into the barracks and turns the lights on calls the room to attention when the officer enters and is then seen to be standing at ease. If he were giving the order, he too would have been standing to attention.
Various streets in the film show modern lamp posts with high-pressure sodium lighting. Both did not exist in 1971. The would have been either mercury lamps or older electric/ gas lamps. Until the mid 1970s, most laps would have been housed on older trolley bus poles.
In the initial chase scene where Gary runs from the shooter he is fired at 32 times (including the first kill shot) from what seems like just one man's gun, we don't see the younger boy fire his gun at all. We also don't see any reloading as they are running at breakneck speed. This would be impossible from a small 1960's era 9mm Semi-automatic pistol which have at most 13 rounds in the magazine.
On a few occasions, the Corporal refers to the soldier's weapons incorrectly, he calls them 'Guns'. The weapon used is the L1A1 SLR (Self Loading Rifle) which has a rifled barrel. Anyone in the British Army would have referred to it as a rifle and not a gun.