Company refusing to pay rent and want to break from agreement

Rent Arrears

I have a leasehold flat in a large block in London which is rented to the company that owns the block. They have their own letting and management office on site and provide short term rentals. They sub-let my flat on short term basis to visitors and also manage everything (maintenance, cleaning etc).

The agreement we have is a short written document between myself and the company in simple English (not a full legally drafted contract or AST) for a 2 year period with no break clauses (they had specifically wanted it that way to ensure that they had a long term assignment). This has worked successfully previously without any issues.

The current renatl agreement is until June 2021 however in Oct 2020 they sent me an email saying that they could not rent the flat out and therefore could not pay the rent and wanted a significant reduction in rent that was due. They have also asked me to find another tenant in the meantime to reduce their exposure which I have now initiated (to mitigate my losses). Obviously finding another tenant will take time and almost certainly will be at a lower rent than the current agreement.

Whilst I am sympathetic to their situation they are a very large, well-funded company and I am dependent on the income from the flat to cover service charges, mortgage, living costs etc.
I have told them that they should seek government help. I have also explained that there is no break clause in the agreement and they are responsible until June 21 or until I find another tenant and that they are still liable for the rent which must be paid which they are refusing to do. I have sent various emails, letters/notices which they are ignoring.

I’ve read a lot about rent arrears when tenants are individuals and under an AST however, i am assuming that that falls under different rules as it is a company and contract.

Any thoughts/recommendations on how to deal with the situation.
Also any recommendations on a good property lawyer? I guess if they don’t comply the only option is the small claims court.