Is cold email legal in the UK?

Yes, for B2B, with conditions. UK cold email is governed by two regimes: UK GDPR (about processing personal data) and PECR (about unsolicited electronic messages). Both have to be satisfied.

In practice, emailing a named contact at a registered company (a "corporate subscriber") is generally permitted under PECR without prior consent, provided you can establish a legitimate-interest basis under GDPR. Emailing personal addresses (gmail, yahoo) and sole traders is treated differently and usually needs consent.

This is a general explanation, not legal advice — for a formal opinion, speak to a data-protection lawyer.