This is the cynical, business-driven reason. The Wonder Pets was a non-union production in the US. When Nickelodeon wanted to broadcast it in the UK, the British actors' union, Equity, pressured the network to use local talent. Rather than pay residuals to US child actors for international broadcast (or face boycotts), it was cheaper to simply re-hire British child actors and produce a dedicated "territorial version."
In the US, the narrator sings fast and high-pitched. But in the , the narrator sounds calm, clear, and speaks with a British accent. They even change "cell phone" to "mobile phone" and "trash can" to "bin." the wonder pets uk dub