There’s advice on landlordzone Google ‘What to do when tenants abandon your rental property: a landlord’s legal guide’
And ‘Tenant Abandonment and the Law’
Also YouTube
’How can you determine if a tenant has left a rental property for good?’ by property lawyer david smith has advice on the specific steps to take (letters to the address emails to all know addresses 2 weeks wait etc etc
Tracing agents for 30 quid would be good to get more evidence -see previous thread Abandonment issues
get eviction specialist advice. If you have legal advice as part of your landlord insurance they should be able to advise?
You don’t say how long they’ve not been communicating or if there’s other evidence they aren’t living there - have you spoken to neighbours? Or if they left keys behind. If there are pre payment utility meters has the credit on these run out? Is there post piling up?
Clearly serving a possession order /s21 would be safest. Not obvious why you wouldn’t do that. The high fines and crriminal offence if you illegally evict aren’t worth risking
Unless you have a reason the keys need changing, dont do it yet- there’s no reason why in the meantime you couldn’t clean the place up if needed, schedule some visits for viewings and give tenant notice of these and do viewings exactly as if the tenant was still there. If you do change the locks document your reason for doing so and be ready to give tenant a spare key immediately if they should return.
You could visit the property regularly in the evening or early morning to check no longer living there and document it. Maybe switch off the electricity at the fuse box then if you return and find its switched on you have evidence tenant has been back. Build up and document the evidence that they have left
You could contact tenant’s employer or guarantor to find out if they have an address to the tenant
If over 30 days notify your insurarers
Good luck