score:7
According to the wikipedia page for the Transport Act 1947, nationalization was effectively a rescue act because the "big four" were bankrupt.
In terms of compensation for their owners, it says:
Shares in the railway companies were exchanged for British Transport Stock, with a guaranteed 3% return chargeable to the [British Transport Commission] and were repayable after forty years.
Whether this was a good deal for the owners appears to be a matter for debate.