Orange and T-mobile have merged in the UK, hence the idea that you could use their towers when you have a t-mobile sim, in the UK.
Which I guess wouldn't be considered roaming, so the first part of my original statement is erroneous. But the second part still stands [ can't force to use orange tower].
"That's it then. Cancel the kitchen scraps for lepers and orphans, no more merciful beheadings... and call off Christmas."