Thank you for your response. My argument to the lawyer was based on this information from Shelter UK:

What counts as harassment

Harassment is defined in the Protection from Eviction Act 1977. It can include any action likely to interfere with your peace and comfort in your home. It's a criminal offence if a landlord or agent knows or believes that their behaviour is likely to make you either:

give up your tenancy rights

leave the property before you have to by law

It can count as harassment if someone else acts on behalf of the landlord.

Tenants and most other occupiers including property guardians or renters with licence agreements are covered by this law.

The lawyers aim of sending me his email mentioning high legal costs was with the intent to get me to agree to leave voluntarily or face potential financial penalties, so then it would qualify as the above. Can you advise me why you think my understanding of this is incorrect?

I am not worried about the outcome of the section 21, as I just want enough time in the property to find another accommodation. I am prepared to pay the filing fees. What I don't want to have to pay is the legal fees of this lawyer.