Trap out, then use a mono bloc box spanner - they usually come in 3's/5's, usually one will fit inside another giving you a long extension box spanner to get over the nut and the other end is below the bottom of the basin.
Not worth the effort by the looks of it, if it's a big nut then it's a basin off job, never going to get anything of significance up behind that basin to grab that methinks.
If it were me:
0. Water off! Both H & C.
1. Remove everything between the tap and the orange connectors, including the orange connectors. You now have two 15mm pipe stubs.
2. Onto each of these stubs, fix a compression isolating valve.
You can now turn your water back on and relax if there are no leaks.
3. Short length of 15mm copper pipe from the top of each valve, cut to meet up with the flexible tap tails.
If the tails end with a 15mm compression fitting, then this just connects to your new pipe.
If the tail ends with a 1/2" BSP nut & washer, use a 15mm straight compression or push fit (or solder!) to attach a tap tail connector to the 15mm pipe (so that the rubber washer is not crushed), and then you can attach the tap tail to that.

Are the pop-up types a real pain to install?
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