Whilst there might be a case for reopening one or two "disused" lines, I think that most of any available money would be better spent on improving the existing network. This would include a steady pursuit of electrification of all but the most minor secondary lines. But first, we need to get abolition of some of the over-zealous regulations about clearances, etc., and a competent and thorough examination of why some projects involving NR seem to be so hideously expensive. Also we need a steady pursuit of extending platforms, so that passenger capacity can be increased, replacing toy-sized trains by something much longer.
Other changes might involve changing the "cultures" within both NR and the civil service. One change would be to contemplate questions like "Is this absolutely necessary" rather than "We must do this". For example, it is really essential to pay for mile after mile after mile of expensive palisade fencing through areas of sparsely populated countryside ?
As for roads - experience is that, within a few years, traffic expands to fill the available road space. More, or wider, motorways just enables more HGVs to pollute our roads, some carrying freight that could be better conveyed in liner trains. Now I realise that no government will adopt this, but I think that the road tax on HGVs should be increased substantially, ensuring that they contribute much more towards the damage (potholes, etc.) that they cause to the roads.
And as someone posted previously, more motorways just create even bigger traffic congestion in towns & cities.