We don't know how he was dressed and if she realized he was a meterman. In the NY area alone periodically there are stories of criminals who have claimed to be metermen who robbed and even raped. It is used as an excuse to get inside. In some of the cases 1 would keep the homewowner occupied while the other ran to the bedroom to look for lewelry and money. In others upon entry they accosted the resident. To scream and yell at an intruder who entered unannounced is quite natural regardless of what you want to pretend. An overreaction would be shooting them in the back as they run away form your property.
I can quite see why, based on your experiences of the NY area, you think screaming would have been a sensible course of action. However, this was a farmhouse in the Essex countryside and from my own experience of farmhouses, the kitchen area is open house and free for all. It's frequently the first port of call for a tradesman because it doubles up as an office. The postman, the milkman, the surgery rep, as well as those with farm business, ALL walk into the kitchen. My friend's family has lived in and farmed from their 15th century farmhouse for generations. As yet, no one has walked into their kitchen and robbed them.
It may be a natural reaction for Americans to scream and yell at someone who walks, unannounced, into their homes, but I can assure you, it isn't what we do. Sheila's reaction was entirely inappropriate and may have contributed to why June told Pam that she was concerned about her.