Having lived next to the WR main line for all of that period, and being a keen train spotter, I can remember seeing no foreign motive power. I think I have a picture of a Type 33 on a Sunday excursion, but never saw any SR engines (or rolling stock) during the reference period.
There were certainly some troop trains in the early fifties, but I wasn't old enough to register the origin of the coaches.
I did once see an LT train being towed through, but have no idea at all where that was going (or coming from!).
The most foreign looking machines we saw were the ROD 2-8-0's (quite uncommon). I can recall one ex USA S160, but that must have been in the early '50s.
Sorry not to have come up with anything! But I think Paddington to Slough (the extent of my knowledge) was pretty much a GWR dominated zone!