H2038 Premises which has not been occupied as the home & Capital Disregards Universal Credit, living with parent, too ill to return to property.

Client was receiving UC, ESA and PIP June 2019-Feb 2023 due to severe Post Viral Fatigue/CFS/Fibromyalgia.

Their Health had improved somewhere in the new year so she took up a remote 6-month temp contract
Finished in August 2023, but has now relapsed with health, Currently on PIP awaiting re-assessment.
Can’t re-claim new style ESA as they haven’t paid enough NI 2 contributions
Refused UC, due to the flat that they own a flat London which they had to move out of in June 19 due to ill health.
Intends to move back once better, so doesn’t wish to sell it. Tenants rent doesn’t cover the mortgage/service charge.
Even whilst working the 6 month contract, required care from parent.

In 2019,  it was accepted that the premises in London was disregarded as capital and yielding income   H2038 regarding Premises which has not been occupied as the home and H2038 of H2 Capital Disregards for advice for decision makers.

On 1st Dec 2023 was refused UC due to capital totals more than £16,000.

Could the above argument about UC still be valid despite it being 3 years ago since they moved back to Sheffield.
They do intend to return to the London flat when well enough, but have only been well enough to work remotely from home with care and support.

If not, would it be worth paying voluntary NI2 contributions for the last two tax years so they can claim New style ESA? (aware that can only be done 6 weeks afterwards, so UC would be preferable) - also previously done this and HMRC questioned why and if they wanted a refund.

Thank you in advance.