Assuming that lines in the Irish Republic may be part of the deal (my "wish" involving same, has also a small UK element) -- as an alternative to my all-English ideas in an earlier post, I'm having thoughts of taking advantage of the "no-strings" offer of 100 miles: to bring back into being in its entirety, the 3ft gauge Londonderry & Lough Swilly Railway. This system conveniently totalled 99 miles (I'll forfeit the balance of one mile) -- running from Londonderry to Tooban Junction, splitting there into routes to Letterkenny and thence Burtonport; and Buncrana and thence Carndonagh. Lines through much magnificently wild and beautiful scenery, doing some spectacular things to traverse same.
The system's first three miles or so out of Londonderry: in Northern Ireland -- then crossing into the Republic (County Donegal). I'm taking it that the magical processes which allow us to reinstate abandoned railways, no matter what physical obstructions may now be there; would equally handwave-away any problems which might be attendant on lines crossing the border in Ireland, in the wake of political nonsense arisen in the past few years.