It is implied Lady Margaret Thompson is an expert metalworker / welder as part of her sideline as a sculptor. However, during the sequence where she is cutting with an oxy-acetylene torch, her PPE consists of a pair of welding goggles and a leather apron: her shirt is still open at the neck and vulnerable to metal splashes. She has also neglected to provide Terry (who is watching) with any PPE whatsoever.
In actuality, when Terry is stopped by the Whiteley's store security, although he had walked to the top of the exit steps, he as not LEGALLY left the premises. hence there would not be any valid case for shoplifting. Had he walked down the three steps onto the pavement then there would be case, but, as it was filmed, legally, any such case would have been thrown out of court any competent lawyer or police officer would have known this.
The Ford Capri that Terry is driving with a 1977 number plate (SLE 71R) is a Mk1 model that ceased production in 1974.
By 1979, the sales, storage, transportation and distribution of goldfish in plastic bags at fairs, in pet shops or anywhere else, had become strictly illegal as the slightest bump or knock to the bag transfers straight through the water to fish as a severe blow. So, not only was the dialogue about the fish incorrect, but even the film crew and the actors were themselves, breaking the laws on animal cruelty in real life.
In terms of Arthur's scheme with the goldfish, he is instructing Stevie to fill a bag with water and then reach into the fish tank, pick up a fish and place it in the bag of water. There are easier ways to get the fish into the bag:
1. Open the bag, put it into the fish tank and scoop up the fish and water into the bag.
2. Fill the bag with water, then use a ladle to scoop up and load the fish into the bag.