You need to look at the forms and decide which option to take.
You cannot get married unless there has been a Decree Absolute so yes you will need your divorce papers
The Embassy website gives this information
Getting married in Tunisia
For information about documents relating to marriage if you are living abroad, such as a certificate of no impediment (CNI) and affirmations or affidavits of marital status, see getting married abroad.
If you are living in the UK and are planning to get married in Tunisia, you’ll be asked to provide a certificate of no impediment (CNI) to prove you’re allowed to marry. You can normally get a CNI by giving a notice of marriage at your local register office or registrar in the UK. Find your local office or registrar in England and Wales, Scotland,Northern Ireland, Isle of Man, Jersey or Guernsey. A CNI issued in Scotland is valid for 3 months. A CNI issued in England, Wales or Northern Ireland will not expire. Check with your local register office to find out how long a CNI is valid if you live in the Channel Islands or the Isle of Man. You should also check with the local authorities in Tunisia to find out how long a CNI is valid under local law.
You’ll need to exchange your certificate of no impediment issued by UK registrar for one that’s valid in Tunisia at the British Embassy in Tunis.
The local authorities may require additional documents such as your birth certificate or evidence of the termination of any previous marriage. You should clarify this with the local authorities.The names on all documents you provide must appear exactly as they do on your passports - if not, the authorities may refuse to allow the marriage to go ahead. You may need to provide evidence if the name on your passport is different to your birth certificate (eg marriage certificate or deed poll).