Please name me an easily accessible chilli pepper which is low heat, sweet and aromatic?
UK stores do not have much variety of chili powder in the low heat range, however you can get good quality paprika in almost all stores. Paprika is low heat, sweet and aromatic (provided you get decent stuff). Some stores sell Spanish Pimentón, which is good quality paprika. Pimentón comes in dulce (sweet) and picante (spicy) although picante isn't really very spicy. 

I have found chipotle powder from specialty stores which is also a good choice, it's basically a smoked Jalapeño. I have found Ancho as well, which is red poblano, neither is particularly spicy. 

There's a lot of chili powder on offer in the UK that is in the hot range from Indian, Pakistani and Caribbean influences, so many stores have sections for each cuisine, it's worth having a trawl through there to see what you can find as occasionally you'll see mild chili powders for sale, and they are good value.