When your house was built there should have been wall ties every 4 courses at opening jambs as this is a weak point in the wall.
When box frames were used the inner wall wasn't returned as the frame closed the cavity. Your's looks like the frame sat behind the external wall which formed a recessed jamb. That's why the inner wall has had some timber to make up the gap.
I've rebuilt loads of jambs in this situation to return the inner wall, but the wall needs ties to the external skin as the inner skin would be separated from the external by a strip of 6 inch DPC.
On modern houses you only have a bit of plastic closing the cavity.
That is a very early cavity wall, possibly they didn't know what they were doing when they built it! A few remedial ties would steady it up and then it should be good for another 100 years. The original windows probably helped hold it all steady.
Thanks for the response guys, this is exactly what I thought. The downstairs section of the same wall had the cavity open in the same way when I was re-plastering (the wall was solid though), I could see all the way down the cavity in to next door. I could also see brick ties that had rusted to nothing, so they had been used, They looked like these top ones:
Would these remedial ties be alright: