The B508 ran around the northern side of Regent's Park in central London.
The road started on the A5 in Maida Vale and then headed northeast along St John's Wood Road. It passed Lord's Cricket Ground before crossing the A500 (now A41) on the corner of Regent's Park.
The B508 continued along the edge of the park along Prince Albert Road before ending on the B505 (now A4201) on the edge of Camden.
The road has since been upgraded to Class I status, becoming the A5205. It seems that the renumbering took place early on since this number fits into the original system whereby four-digit roads in central London would be numbered Ax2xx.