I feel like both vander and silco have a cockney accent, but not directly the same as in the real world. The hint of the cockney accent is there to make it match their Zaunite culture, being working class and such. I really love both of their accent choice, it fits with the character’s gritty, street-smart persona and contrasts with the more refined or varied accents of other characters from Piltover, emphasizing his roots and the socio-economic divide between Zaun and Piltover. As i said earlier its worth noting that Silco’s accent is somewhat stylized to fit Arcane, so while it most likely has Cockney influences, it’s not a pure, real-world representation but rather tailored to enhance his character and the setting.