OP - What exactly is your business? Do you pay your landlord (the son and daughter) a small rent and then effectively sub-let the annex as a holiday let to paying guests (or sub-let it as a business unit to another small business)? For example, you pay the "landlord" £100 a week and then sub-let the property for £500 a week, making a profit of £400 a week that you then pay income tax on?
On what basis does this agreement operate? Do you have a contract with the "landlord" and if so, what conditions are there - e.g. notice periods, who is responsible for repairs, who pays utilities etc?
It sounds like the accountant (or someone) is suggesting that your business - lets call it "EM Rentals" - should operate in the future on a different basis, and that the current arrangement might not be the best in terms of tax efficiency or inhertance tax planning.
Alternatively, the "landlord" now wants to control the business and keep a larger share of the profits for themselves. Perhaps your "landlord" wants to take over "EM Rentals" and collect the rent directly into their bank account and cover the operating costs and then pay you whatever surplus there may be. In effect you become an employee (presumably you would still actually do the work of finding tenants, sending invoices, handling queries, organising cleaning etc) but with no guaranteed income. The "landlord" could easily add all sorts of fees and costs into the operating overheads so that there is effectively no profit left for you to take as a wage - which is clearly not a viable option for you.
Questions:
What form does your business take - is it registered as a sole trader, limited company or ..?
Are you the owner/director of "EM Rentals" and do you own the business name, the website address and phone number, the customer list and the goodwill of the business? If so, what is all this worth?
Is there a contract between your business and the "landlord" - particularly around notice periods? Rather than just collecting the rent themselves (or having you pay it directly into an account that they control) is there a notice period? Who do the tenants have a contract with (presumably your business) and who will they pay (or who have they already paid) the rent to?
Does your business have any other activities apart from those linked to the annex - e.g managing other rental properties? If the "landlord" took over the rental of their annex, would that be the end of your business?
Whatever the answers are to these questions, you should be asking the accountant and/or the "landlord" to put in writing what they are proposing and why they are proposing it - and then take that document to a different solicitor or accountant for an independent opinion. On the face of it, it would appear that your business is being taken from you without your consent.