We see a video of Rebecca's VA in the recording booth (coupled with Yvonne Grundy, Nia's VA) and from looking at this lady, she has strawberry blonde hair with blue eyes, which exactly fits Rachael Miller's description. I guess why I thought so as well was because Rebecca's voice sounds similar to Alice from Angelina Ballerina: The Next Steps, who Miller also voiced.
That reminds me, you should try editing the Nia and Rebecca sprites on DA to edit the prototype liveries onto them. Also, I can actually reveal that Rebecca's number, NWR lettering and nameplate all use the font from the Thomas & Friends logo. Can't remember what it's called, but I can easily notice it's the same font as the logo.