Tenancy ends and how to observe Covid-19 lockdown?

1. Guidance states moving is prohibited. So 2, 3, 4 are irrelevant. But assuming you ignore this:
2. To achieve social distancing, I suggest you let her leave and do the check out after. Is it essential- depends on interpretation of local police! Are you likely to be stopped? If not, don't worry about it.
3. Yes, but you get vacant possession on the day she checks out.
4. Return only after you both agree on any delapidations. Get a signed agreement. Process depends on whether it's a DPS "insured" or "custodial" scheme - go to their website for details.

3. When she registers as living somewhere else, the LL becomes liable for the C Tax, even if her stuff is still in the flat, hasn't returned keys etc. etc.
Liability due Local Govt Act 1992 Sect. 6 is the person falling within highest level of categories:-
a) resident freeholder b) res. leaseholder c) res. statutory or secure tenant d) res. licencee) any other type of res. and f) owner.
I always argue (to Hartlepool) that the tenancy hasn't ended. In the last one, the months notice hadn't even expired, as well as no keys, and possessions still in situ. If tenant chooses to have two tenancies current simultaneously, thats their choice. Of course it doesn't get me anywhere.
4. I would think DPS would instinctively side with the tenant too. As you know, dep. has to be dealt with within 10 days of termination, and I would be arguing that vacant possession (stuff removed, keys returned etc.) starts the clock ticking.

I would suggest that she can't have her cake and eat it ie keep her possessions at your flat and not pay rent whilst using the room as storage. I suggest that she ought to rent a van with another member of her current household and drive to the flat, removing her possessions without interacting with the house mate, or have the other housemate. Alternatives would be that the housemates packs up her clothes into bin bags and puts these outside at the agreed time and that the furniture is given to you or agreed to be collected and put into local storage (arranged by her).
Assuming that they are housemates and not separate flats, then the council charge would have been split between them both, so the other tenant will now have to pay 75% of the total bill with the single person discount, same for the other utilities which will revert to the remaining tenant.
With this in mind, does the other tenant wish to stay or leave? Because if they both leave, you will get vacant possession and all the bills, but you would then be able to deep clean it, take a video tour for prospective tenants and re-rent without an issue as it is an empty house and people can move in without restriction.

If the tenant is still in possession of the room ie has her belonging in the property then she is liable until vacant possession is obtained.

The rest is irrelevant.

As for Gunga comment about council tax, make sure your tenancy states them at they revert to ‘contractual Periodic’ at the end and NOT ‘statutory periodic’ as the tenant will be liable to the next tent date. It’s not much but it all helps

Many thanks all. Much appreciated.

The tenant has agreed to pay 1/2 the council tax and all utilities and services in exchange for paying her April rent in full and no rent from 1st May until she vacates her room and removes her possessions and furniture. Phew.

Yes a relatively good outcome if she lives up to her promise. It does illustrate once again how we LLs are held to exacting standards and levels of compliance, often on pain of disproportionate retribution, whilst we are expected to bend to accommodate those who don't think the rules apply to them.