I recall an couple of articles in the 1980s, one by Howard Geddes and one by Peter Tatlow about the Lms northern division, ex Highland but the information applies throughout the UK. From a photo or two of Inverness, in Lms days the proportion of wagons were Lms - 5, L er - 4, GWR -3 and Southern - 2. From photos of Blair Atholl and Kilwinning ex g&SWR). When I was looking at a similar era at Basingstoke it was proportionally the same - Southern - 5, GWR 4, Lms 3 and lner 2. There would be some variations locally, especially if another railway was nearby. Private owners wagons- think what local businesses would do - the Highland railway got coal from collieries in Fife, so don't have lots of south Wales colliery wagons if you want to be strictly accurate. Remember too that in the 1950s and after, private owner wagons were absorbed into British railways and lost their liveries.