Concrete panels slid into the slots. Off the top my head these could be Maycrete or British Concrete Federation huts designed in 1942. This camp was, I think, originally built for the Bolero programme , but may have been occupied by British troops before the US arrived in numbers.
yes you are right it was originally built for the US. My own thought is as you have said - Maycrete Hut - BUT whilst i have seen loads of them from intact to ruin to being demolished, i have never come across one left like this the upright and no roof beams
Most of the building has been 'recycled'. Panels useful to farmers to fill up farm track potholes/soft spots. Heavier plant needs old tracks beefing up especially given the wight carried in modern trailers. There is quite a trade in old concrete railway sleepers laid reversed where I live.