There are buggies available that are marketed as only suitable for 3 month olds—some only from 6 months—and up. The reason is that they don’t have 180-degree lie-flat seats, and it is important that newborns lie flat for healthy physiological development.
If your newborn is propped up in a sitting position—or even tilted at an incline—his tiny body scrunches up and his breathing is restricted. Lungs are the final organ to develop in-utero, and so it’s vital that the respiration is not impeded in any way; their lungs are often still developing.
Another more obvious reason to keep your newborn in lie-flat positions is to support proper spinal growth; and also to assist in muscle toning. And remember that a newborn has no neck control—lying flat until they can hold up their heads will prevent them flopping forward in uncomfortable positions.
Lying flat allows your baby to reach feet easily for exploration, and, last but certainly not least integral, your newborn can lock onto your face and follow your movement without having to search upwards to find you.
Be aware that European and British buggy standards are not the same; a buggy marketed as suitable from birth in the UK could only be marketed for 6 month olds and older in Spain—given the benefits of lie-flat models, this is possibly a case for updating British trade standards.