It's a scam.
An EA tried that with me, and I said 'please provide me with the invoice for the pre-tenancy professional clean and I will happily do so.' The previous tenant, who I met, left it clean as a whistle! They backed off.
It quite often comes up in housing related scam articles.
However, I've just discovered it's in fact illegal.
I would just say 'insisting that tenants get a property professionally cleaned has been illegal since 2019. I suggest your letting agent updates the advice they give landlords and which clauses they can and cannot put in their leases to reflect this.'
"In the past, some landlords included clauses in their tenancy agreements requiring residents to hire a professional cleaning service to ensure their property was returned to its original state when they moved out.
This is now illegal under the Tenant Fees Act and you can be fined up to £5,000 for including such a clause.
Of course, you can advise tenants to use a professional cleaning company before they move out. But whether they choose to do so is up to them.
You can also choose to use a professional cleaning company yourself for peace of mind. However, you cannot automatically charge your resident for this."
https://evo-pm.com/insights/end-of-tenancy-cleaning-laws-uk-a-guide-for-housing-associations/
"The Law Is Clear: Professional Cleaning Clauses Are Banned
The Tenant Fees Act 2019 was designed to make renting fairer, stripping out unfair costs and hidden charges. One of the biggest changes is that landlords and agents cannot demand tenants pay for third-party services, including:
- Professional end of tenancy cleaning
- Inventory checks
- Gardening services
- General admin or processing fees
If your tenancy agreement includes a professional cleaning clause, it’s unenforceable. And if you try and enforce tenants to have the property professionally cleaned, you could be breaking the law.
The risk: Landlords or agents who insist on professional cleaners could face fines of up to £5,000 for a first offence—and unlimited penalties for repeat breaches."
https://www.360-properties.co.uk/news/why-landlords-cannot-force-tenants-to-pay-for-professional-cleaning-ninw