I have seen nothing yet to negate that above.
I'll do my best to explain why I think your intended approach is flawed:
The regs document BS7671 has only one reg about ring final circuits and honestly it's pretty vague, amounting to use one of these protective devices, and this cable, and make sure you won't exceed the current carrying capacity of the cable for long periods.
To complement this there is an appendix 15 in BS7671 specifying ways to comply with the one regulation (433.1.204)
That says that the requirements can generally be achieved by (Fig 15a ii ) "
Not supplying immersion heaters, comprehensive electric space heating or loads of a similar profile from the ring circuit." (direct quote, page 555)
Indent iii also advises to run dedicated radial circuits for various large loads over 2Kw.
To further complement the regs there is the on site guide. If you follow this to the letter you are likely to comply with the actual (more complicated) regulations.
As you have stated, the OSG also says to put immersion heaters on a separate circuit (paraphrased)
It makes no sense at all to interpret the OSG as you suggest as it would completely undermine appendix 15; the power is still coming from the same ring final circuit along the same cables, and the stated intention is not to exceed the ring final circuit's CCC for long periods.
The actual compliance comes down to the loading of the circuit, and it may be safe and comply today. It also may not be safe another day if the heating fails and the fan heaters come out.
This all amounts to a choice. You either do an installation that follows the advice in the regs and the on-site-guide that any competent electrician won't question, or you do something unusual that future electricians will do a double-take at, instantly think of the sentences in black and white in two places that advise not to do this, and get them wondering if it's compliant.
Even if you go ahead, I'm intrigued what your intention is for controlling it? Would he have to go upstairs when it's sunny and turn it on?