Why did the Celts begin their invasion and conquest of southwestern Britain around 450?

The Celts began their invasion and conquest of southwestern Britain around 450 AD primarily due to socio-political changes and pressures in their home territories, including population growth and the search for new lands. Additionally, the weakening of Roman influence in Britain following the decline of the Roman Empire provided an opportunity for Celtic tribes to expand. This period marked a shift as groups sought to establish dominance and control over new regions, leading to increased conflict and territorial claims.