SNCF, to my knowledge, never had spending decisions made in Westminster. It just puzzled me, knowing very little of the historical reasons, that, gauge and Irish Sea aside, there weren't closer ties with British Rail. Economies of scale and best practice and stuff.
When Northern Ireland was created it had a system of government called Home Rule. This in practice meant that it was virtually an independent country, with only things like currency, defence and foreign policy in common with the rest of the UK. Many things were different about NI before the Troubles: an armed Police force, the requirement to own property or have your name on a rent book in order to vote, abortion being illegal (still the case), blatant discrimination on religious grounds in terms of jobs and housing, to name but a few. The province had its own Government in its entirity, with a Prime Minister, Cabinet and all the trappings of a nation state.
This Home Rule was the model intended by the British Liberal government for the whole of Ireland before the First World War. In the event two Home Rule states were set up, Northern Ireland and Southern Ireland. It was inconceivable that Southern Ireland could have been set up with the same relation to the UK as Scotland or Wales, and the very arms-length set up required for the South was also applied to Northern Ireland. In practice the Home Rule state of Southern Ireland immediately transformed itself into the Irish Free State on partition, and finally severed all remaining links with Britain in 1949 (left the Commonwealth).
All this means that laws and legislation passed in Britain did not apply in Northern Ireland unless specifically brought in by the Northern Ireland government. Things like the Welfare State didn't come in in NI until later than in the rest of the UK. Railways in Northern Ireland were indeed nationalised in 1949, but the model of the UTA bore more resemblance to CIE in the Republic than to BR. For a year between nationalisation in Britain and in NI the former LMS NCC lines were actually part of BR, as was the NCC's joint share (with the GNR(I)) in the County Donegal Railway, whose lines were almost entirely in a different country, but the NCC was absorbed into the UTA shortly after its creation. The GNR(I) was split between the UTA and CIE in 1958, resulting in the closure of nearly all of it because many of its lines crossed the border in several places, and once the UTA had done its dirty work closing sections in Northern Ireland, there was little CIE could do with isolated sections in the Republic other than close them as well. This is why there are virtually no railways in the counties along the border.
Following the abolition of the Northern Ireland government in 1972, NI did become more closely linked with the rest of the UK, but by then the structures of ownership of the railways and other public bodies was established and were different from Britain. In short, Northern Ireland's spending decisions on such matters were *not* made in Westminster.
