This is not a typical RAMJET or SCRAMJET
https://en.wikipedia.org/wiki/Scramjet

From ReactionEngines’s web site:
http://www.reactionengines.co.uk/sabre_howworks.html (2-minute embedded video)

“SABRE is the first engine to achieve this goal by operating in two rocket modes: initially in air-breathing mode and subsequently in conventional rocket mode:

“- Air breathing mode – the rocket engine sucks in atmospheric air as a source of oxygen (as in a typical jet engine) to burn with its liquid hydrogen fuel in the rocket combustion chamber

“- Conventional rocket mode – the engine is above the atmosphere and transitions to using conventional on-board liquid oxygen.

“In both modes the thrust is generated using the rocket combustion chamber and nozzles. This is made possible through a synthesis of elements from rocket and gas turbine technology.”

Reaction Engines’ SABRE engines ingest O2 while in the atmosphere in order to feed rocket engines, then convert to on-board O2 tanks once the atmosphere becomes too thin.

In the SABRE design, there is compression at the air intake, but they may be planning to simulate this on the ground, or perhaps they intend to use wind tunnel testing to simulate the high speeds at altitude. They are not clear about the test plan, the test facilities, or the test design.