The two most common varieties marketed in US grocery stores as "Italian Sausage" are hot and sweet (or, depending on what region of the US, mild,). The main difference between hot and sweet (mild) is the addition of hot red pepper flakes in the spice mix of the former.
Finding Italian sausages in the UK
In the UK, Italian sausages are often marketed Sicilian style, Italian recipe or as justItalian sausages. If you cannot source any of the former, then look for a coarsely ground pork sausage with fennel or a pork sausage with added chillies. Toulouse style sausages would also work as a substitute.