Kitchen needs to be inspected by the environmental health department of your local council.
You need a basic food hygiene certificate (you can do it online here www.food-certificate.co.uk/.
You need to register with the tax office as self employed.
Go and talk to your bank about financial support, possible a loan for equipment etc. Also possibly an accountant to figure out what you can claim as tax deductable etc.
Make sure you know your market, do research for other cake businesses locally, figure out what people will pay for a cake (that allows you to work out your profit margin, one paid for materials, gas/electricity etc etc).
Look to getting a website (take good quality photos of your cakes), also go to www.vistaprint.co.uk/vp/ns/default.aspx?xnav=welcomeback&xnid=removecookies for cheap business cards, stationery etc).
Be aware if you figure out what your hourly wage is for making a cake that is priced so people will buy, it is pretty dismal. I do it because I enjoy it, I don't actually need to work (am very lucky in that respect), and it provides me with a bit of pocket money.
To make it a viable business with real earning potential is hard hard work.