To answer this question we'd also need to consider some quite radical changes to how the UK is governed. Despite devolution to Scotland and Wales as well as some more piecemeal devolution deals to some local authorities in England, the UK is still a highly centralised state in comparison to many other western countries, particularly European countries. All larger EU countries (France, Spain, Italy, Poland) have some form of regional government, with Germany of course having federal states. These have far more responsibilities for transport than most parts of the UK, being directly involved in rail service provision.
Below states, units of local government in the EU tend to be smaller both in terms of geography and population than the UK, where the direction of travel has been towards large unitary authorities. However, the various district , provincial and municipal government have a lot more responsibility for local transport (bus, light rail etc) than pretty much anywhere in the UK . Largely because central governments in other countries have resisted the urge to effect apply a one size fits all approach by national legislation (1985 Transport Act, for example, ironically simultaneously an act of deregulation, privatisation and nationalisation all in one).
Devolution to Scotland and Wales has essentially just created a pair of mini-Westminsters, with power not being devolved downwards from Edinburgh or Cardiff. In Germany it is probable the likes of the South Wales Metro would be led by a consortium of local authorities via a transport association rather than a state government.
So as a starter for 10, if we were handing over planning to individuals from elsewhere, I think the concept of regional autonomy within England would be revisited, with significant responsibility in key areas such as local and regional transport removed entirely from the UK government. It is also likely the 1985 Transport Act would be repealed so as to enable local authorities either individually of via joint transport authorities, to plan bus networks, either by operating them directly or (as is common in Scandinavia) tenders/franchising. From a rail perspective, regional authorities would be specifying and funding local and regional rail. The operators would either be a national publicly owned operator, a regional publicly owned operator or a privately owned but contracted by the region.