is there a list anywhere of categories I can legit claim as a business expense?
A good starting point is the tax return itself which gives you the headings you'll have to fill in.
But, every business is different, so something may be allowable for one, but not allowable for another. Best to think of the main rules first rather than individual headings.
It's all about motivation and actual use of whatever you buy. If you buy something purposely to sell it, then the cost is clearly allowable.
If you need to buy something needed to run/grow the business and don't use it for private use, then again, clearly allowable, such as a computer, advertising, website, business insurance, train fare to visit a client, etc etc.
The problem areas are the "dual purpose" areas where a cost is incurred partly for business and partly for private use, i.e. when you use part of your home for business, or you use your car for both business and private journeys. You have to apportion costs.
Then there are things which are specifically non allowable such as gym memberships, children's nursery fees, your own private healthcare, etc.
Most accounting software will come with a chart of accounts which gives a good range of headings to give you an idea/reminder of what typical businesses would claim for.