a big question about the environment at that time

[Brennus]

Regarding horses in Britain, the wild species (Equus ferus ferus) appears to have been hunted to extinction in during, or shortly after the last ice age. Horses (Equus ferus caballus) were reintroduced to Britain during the Bronze Age. The lack of juvenile horses on some British Iron Age sites has led to the suggestion that feral herds, similar to the modern Dartmoor pony, were used a source of horses. Selective breeding of horses did not occur until the arrival of the Romans.