Hi Sugar Rush, the first thing you need to do is contact your local environmental health officer and register with them. Every area is different, some simply require you to register your details with them as cakes are considered low risk, many will want to do an inspection, and there may be some small changes you need to make to your home kitchen or way of working before you can begin. You need to register 28 days before you start trading. You will need to do a basic food hygiene certificate, you can do this at a local college, or many places now offer online courses for £20-30, it only takes a couple of hours. The only other thing you must do is to register with the inland revenue as self employed, even if it's only going to be part time in addition to your main job.
You may wish to consider insurance in case of damage to equipment, loss of earnings etc, although this isn't a legal requirement. You will need to check with your home insurance provider that running a business from home won't void that insurance. If you plan on offering a delivery service you will need to add business use to your car insurance, and if you plan on selling at markets, fairs etc you will need public liablity insurance.
Hope this helps, good luck with your venture and my biggest piece of advice would be to properly work out ALL of your costs BEFORE you begin, and don't forget to include a realistic amount for your time. If you are on facebook, check out the cake forum - a friendly group of uk bakers who are all happy to help each other.