To provide you with an accurate answer, I would need to conduct market research specific to Wiirral. However, according to a study conducted by Just Eat and YouGov in 2017 on UK takeaway ordering trends, the most popular sandwich fillings were chicken (36%), bacon (34%) and ham (28%). As for customization, customers often request extra ingredients like cheese, salad, mayonnaise, or sauces. Unfortunately, I don't have specific data on Wiirral but these general trends should give you a starting point. To get the most accurate information, it would be best to reach out to local sandwich shops and cafes in Wiirral and ask them directly about their customers' preferences and common customizations.