Notice given by whatsapp is perfectly acceptable and always was - just like an invalid notice given by tenant (eg not long enough vs what tenancy agreement says) becomes valid if the LL accepts it. Pretty sure there is caselaw on this sort of thing. @david122 may know or good solicitors would
See
Sounds like Landlord Action have been unduly cautious and cost you a lot of unnecessary £ in expenses time and lost rent.
There is nothing to stop you accepting the whatsapp notice now and RRA says notice can be given that way (tho might or might not be a legal right of the tenant to do that as was before 1 may 2026)
You then dont need to bother with bailiffs etc
I would gather the evidence of abandonment (possessions removed, rent stopped, mail piling up, no change from week to week in electric and gas meter readings) plus whatsapp notice from tenant, and ask Landlord Action to tell you what if any caselaw they can find saying A. Tenancy continues and B. Tenancy has ended as abandoned.
And then ask them to justify why they still think bailiffs needed. I think they have given you very poor advice
Best