You can only email people if you have a legitimate reason to. The law states that people have to "opt in" to receive mails from you.
However- this is a bit of a wooly law, as opting-in can literally mean they click a button on your website that adds them to your mailing list. However, there exists a "soft opt in" which means you obtain their contact details in the course of a sale or negotiation for the sale of a product or service to that person- and even then- you *must* give people an option to "opt-out" of emails.
You can't send emails to people with whom you've had no contact before. "In the absence of express consent or consent through the ‘soft opt in' option described above, you are not permitted to send marketing emails to individuals"
In a nutshell- email them if you've dealt with them before, give them an option to opt-out, but don't email them if you've had no dealings with that individual at all (this might happen if you've been given a list of emails from another person, or you've bought a list).