What actually breaks dormancy
The test is significant accounting transactions — entries that would appear in the company's accounting records. The law forgives exactly three: the shares subscribers take when the company is formed, fees paid to Companies House, and late-filing penalties. Everything else counts.
Bank interest is the usual dormancy-killer
Two separate tests
Companies House and HMRC each decide dormancy their own way. You can be dormant for your accounts and still be active for Corporation Tax. Clearing one doesn't clear the other, which is why a "dormant" company can still collect a £100 penalty for a CT600 it didn't know it had to file.