The airbrucks appear to be above ground level so water will not be running in through them unless there are several inches of flooding round your house.
If there is water in the subfloor void it is most likely coming from a leak, of either a waterpipe, or a drain.
A 1930s house is likely to have both, or either.
Have you got a water meter? Have you got access to young, undamaged ears?
A plumbing leak might be from the buried water supply pipe, which probably runs in or under the floor, between the outside stopcock, and where the kitchen sink used to be even the house was built, then onward to where it is now. If you have solid concrete floors in any of these areas, examine to concrete to see if it is damp. Otherwise, take up some floorboards, sniff, feel the subsoil and look at it with a torch. Do you have a water meter? Do you know where the outside and inside stopcock are? I can explain more. I expect your supply pipes are iron, so the first leak is probably at an elbow where it turns towards to come up through the floor, or at a joint, especially where pipes were extended to a new kitchen.
If it is a radiator pipe or hot water pipe you will (should) have noticed the shortage and constant topping up.
Your drains and gullies are salt-glazed clay, like a teapot. IME they are always cracked or broken and leaking in the ground, and have been since 1940. This is usually evidenced by the gullies having subsided below their original position, and multiple fruitless efforts to hide (they do not repair) the gaps with patches, mortar poultices, and concrete filling. Look for these signs.
Less commonly, water comes from nearby streams, or runs down a hill if you are on one. It might come from an adjacent neighbour's leak.