By "underestimated the sizing" please be specific about whether the Raleigh is too large or too small.

I'm going to assume too small.

Yes, vintage sizings are real measurements because the TT is generally level.
On compact frames the TT is not level so they measure to wherever they want - sometimes the TT, sometimes the top of the ST (which is cut at an arbitrary length)

The TT also has a 1" diameter on the Raleigh, a modern frames tubes are generally much larger, so even if the modern TT is level, the effective height measured c-t will be larger than on a vintage bike with the same c-c measurement.

You can run a longer seatpost. Just make sure you have it inserted above the minimum. You may end up with quite a bit of saddle to bar drop. Whether you like that or not is personal preference.

You may also find that the TT is too short on the Raleigh.