I'd agree with @travelallthetimeon that list but would also add in Personal Travel Agents at mid counties. You need to have a host agency with great commercials to be competitive (and it is an extremely competitive industry), so backed by Hays, Advantage, Mid counties coop etc.
The only point I'd disagree with from some PPs is that you don't have to be ABTA - Travel Counsellors aren't and neither are the likes of Trailfinders as a retailer. You do need robust financial protection and that can be from a trust account which IMHO is better than ABTA, but I appreciate customers know ABTA.
If you have money and time to invest a franchise with Travel Counsellors or someone like Not Just Travel or Holiday Franchise Company will bring you a sustainable business, but they are expensive (for a reason).
If you're going down the side hustle route I'd beware, it's really not as easy as FB recruiters for the likes of inteletravel make out. The booking is the easy bit, but what happens if you're at your full time job and there's a major issuing resort, a weather incident, natural disaster or even terrorist attack? Good travel agents work insane hours to help their clients in these circumstances, if you can't do that your reputation will be gone. I'd also think about who your customers might be, and why they would book with you rather than another local, experienced agent? If they're price led they'll book online, if they want bespoke service they'll want a professional.
I'm not trying to put you off, it's a brilliant industry to work in but not as easy as it might seem! FWIW I'm not a travel agent but work in the industry across all agencies so have quite an independent view. If I was going to be an agent I'd probably go to Designer Travel or ITE.