The bigger the cheques and the quicker they are written, then the faster a locomotive can be built. It is that simple. Here in Britain there is no shortage of skills, experience and workshop capacity to build steam locomotives. The only limiting factor is funding. If someone could write a cheque for a million, you'd have a steam locomotive delivered before Christmas.
Its not just Brecon Mountain Railway, by the way. Here is a 2' gauge Baldwin built in Gloucestershire for the Lynton & Barnstaple Railway
L&B Lyn
We have literally 7 or 8 workshops which are actively building new steam locomotives, from little 0-4-0's up to a standard gauge 2-8-2. There's a couple of pacifics underway, several 4-6-0's. Most of these are amateur builds, using professional services to assist, but there are 3 or 4 companies who build as a commercial proposition.