Paul Weaver wrote:

The road you're talking about IS entirely residential
and I expect that is why it closes to straight-on traffic at night

The road is a green "Primary Route", a major thoroughfare for traffic
from the north of London to the south.

There are several symbols defined (dot in road, line across road,
yellow/purple dashes) to indicate a road with restricted access.

In the A-Z I have (about 2 or 3 years old) it's shown in purple (as a
restricted access road, like Oxford St). Looks a bit odd having a
purple section in the middle of an orange A-road, but there you go...