Some for puppies! Especially if it’s something that can be chewed or licked for a long period of time. Things to put into a Kong or a licki mat! Love the other persons idea of making them too, I bet that would go down well selling locally.
On setting up a small business, first think up your product/s thoroughly.
What is legal (and ethical) to sell, do you need any license?
How will you get your product to your customers?
Are you willing to post, or would you rather set up a stall, or both?
Lots of banks will let you set up a business account for free, it’s not legally required but I would advise it as it will keep your books much easier to manage.
Advertising : Facebook and Nextdoor if you are keeping it local, along with friends and family. Facebook will let you pay to advertise your product in your local area if you set it up correctly. It can be amazing for businesses as you can tag who you want to target (dog lovers/pet owners)
Go into the GOV HMRC website and it will walk you through self assessment registration as well. You don’t need to do this if you earn under £1000 a year from it but I would say you should do it anyway as you wouldn’t be paying tax on a small amount, and it might get big quickly if you do it right!
Good luck OP! It’s the start of a fun journey 