You need an embassy visa for United Kingdom if you have a Tunisian passport

Travelers need a visa to visit United Kingdom for Business or Tourism. Travelers must obtain this visa from a local embassy, consulate, or visa center before traveling. The United Kingdom is moving from physical immigration documents to eVisas. If you use a biometric residence permit (BRP), biometric residence card (BRC), or visa vignette sticker or ink stamp in your passport to prove your rights, or to enter the UK, check the link to find out what you need to do. Creating a UK Visas and Immigration (UKVI) account to access your eVisa is free, straightforward, and the move to an eVisa will not affect your underlying immigration status. Visa nationals should continue to carry their in-date physical immigration documents with them when they travel internationally, along with their passport. If you have a biometric residence permit or card (BRP/BRC) you should continue to carry it with you when you travel. If you use an eVisa to prove your rights in the UK, then you must tell the Home Office about any passport you intend to use for travel if it is not already linked to your UKVI account. Access your UKVI account by checking the link to do this well before you travel.