The law on sending commercial emails in the UK is very clear.
You can email an individual if they have specifically consented to receiving emails from you – for example, by ticking an opt-in box.
You can also email an existing customer who has bought (or discussed buying) a similar product or service from you in the past – but only if you gave them a clear chance to opt out of getting marketing emails when you collected their details, and in every message.
You can email any corporate body (a company, Scottish partnership, limited liability partnership or government body)
Sole traders and some partnerships are treated as individuals – so you can only email them if they have specifically consented, or if they bought a similar product from you in the past and didn’t opt out from marketing messages when you gave them that chance.
.For a more detailed explanation of the law, go to https://ico.org.uk/for-organisations/direct-marketing-and-privacy-and-electronic-communications/guide-to-pecr/electronic-and-telephone-marketing/electronic-mail-marketing/