Is cold email legal in the UK for B2B?

You can email UK companies. You must not email individuals without consent, except for a narrow soft opt-in, and you must follow UK GDPR.

In the UK, the Privacy and Electronic Communications Regulations (PECR) allow cold emails to corporate subscribers (Ltd, PLC, LLP) without prior consent, but you must still provide an opt-out and follow GDPR rules for lawful processing.
For sole traders and partnerships, consent is normally required. 

About, Services

ICO – electronic mail marketing

ICO – lawful basis

Summary:
Cold email is legal for B2B in the UK if you target corporate emails, follow PECR, give opt-outs, and comply with GDPR.