Mechanical ventilation is not automatically mandatory. Part F allows several strategies, including natural ventilation with extract fans, but in a very airtight modern dwelling a continuous mechanical system — often mechanical ventilation with heat recovery — is frequently the practical way to deliver adequate air change and support Part L compliance.
The regulations are performance-based: the requirement is adequate ventilation, not a specific technology. Where a dwelling is built to a moderate airtightness level, natural ventilation with background ventilators and extract fans can be a compliant solution.
As airtightness improves, relying on background ventilators becomes less effective, and continuous systems are commonly used instead. MVHR also recovers heat from extracted air, which improves the energy calculation, but it must be designed, installed and commissioned correctly and maintained thereafter, otherwise it can perform worse than a simpler system.