If i was allowed more than 20 mil, say at least 50mil, then id put a lot of money into both the Wensleydale and Waverley routes. Get them both fully reopened, and at a decent speed.
Buy a few locos. Mainly 47s. Get them mainline certified, and refurbed up to a very high standard. I want reliable locos!
Then sit back and enjoy a few railtours behind my own locos, and running over the Wensleydale and Waverley!!
Expect freight traffic on the Waverley. If you keep access charges at a reasonable level, the freight companies would bite your hand off for a route that avoids the WCML north of Carlisle. Just make sure you save a bit for the Carlisle avoiding line, you'll need it.
