They do not use any emojis in their emails because this would not fit in with their brand image, they are a more sophisticated brand than PLT so they communicated differently. They also do not send as many e-mails out as PLT do, this is because they do not do as many discounts as PLT and do not need to advertise discounts via emails.
Overall both retailers communicate differently because they need to communicate in a way that will encourage their customers to shop with them. For Pretty Little Thing using slang and emojis will appeal to their target market because they are a lot younger and will be more used to this, so they prefer it.
Whereas Topshop keeps their online communication more sophisticated because they are marketing at a wider range of consumers. However they both use the same platforms to communicate with consumers, with social media and e-mails being very important tools.