Hi, thanks for clarifying matters.
The procedure is as follows-
What needs to happen first of all is for you or the current Tenant approach the Freeholder for consent to assign the Lease to you (the Landlord or his Agent will want to see Bank/trade references for you). Once approved, the Freeholder's Solicitor needs to prepare a Licence to Assign which is the formal consent to the Lease being assigned to you. The current Tenant will be responsible for paying the Freeholder's Solicitors costs.
Whilst this is going on, the Tenant's Solicitor would normally be liaising with your Solicitor as an Assignment document will need to be drawn up and if you are buying the business, a Contract will also need to be prepared to include a list of the assets which are included in the Sale.
Once everyone is happy and signed, you will then be able to complete the purchase.
I do therefore recommend that you contact a local Commercial Property Solicitor to deal with matters- if you go n the Law Society website and click "find a Solicitor" it will provide such Solicitors in your area.
I hope this helps and if so, I would be grateful if you could rate my answer. I will then be happy to answer any follow up question you may have.
All the best