I think that this is what I might do, cut the pipes, use a box spanner to remove the back nuts, and then install a flexi tap connector (I wanted to include a mini-valve into the pipe anyway).
I need to check the sizes, but would something like this be OK?
https://www.screwfix.com/p/flexible-tap-connector-with-valve-15mm-x-x-300mm/3558g
Can I just use a basic pipe cutter, something like this?
https://www.screwfix.com/p/rothenberger-minicut-pro-2-6-22mm-manual-copper-pipe-cutter/71782
I would think it's a male thread on the elbow, so you would need to turn it clockwise as we look at it on the pic.I tried to remove the nut as fixitflav suggested, and I also put a spanner against it, but too high above the elbow, and so I have managed to bend it exactly as he predicted (without shifting the nut at all).
Don't know how you did it, but what I meant about the open-end spanner is put it on the elbow so it hangs nearly straight down, parallel to the other spanner (can't think of a better way of putting it but you know what I mean!). Ideally you set it up so the far ends of the 2 spanners are about 3" apart, and need to move together. Then you can grip both in one hand and squeeze, putting hardly any external force on the pipes. To do that in your case the main spanner is near the wall, the "holding" spanner away, so it might be awkward. If you have a spanner in each hand just try to put equal force on each.
If you can get it apart, you might be able to re-use the elbow. putting in a new tap connector. You might have to do that anyway, even if you hadn't bent it, as the new tap tail may not be same length as the old. Nowadays they're standard, at 50mm I believe, but your old one could be different.