"The Medway Handyman" gurgled happily,
sounding much like they were saying:

What I'd like to know is when the present 30 & 40 limits came into
being - and what braking systems were common at the time?

The default urban 30mph limit was introduced to the UK in 1935.

Hydraulic brakes were a real novelty at the time, and older stuff still
on the roads would still not have had front wheel brakes.

Any data on the braking distance of such vehicles?

No. But it's not going to be even vaguely comparable with anything modern.

Things have come a long way in 70+ years.

Including large Edwardian cars with the brakes full on.