Ignore the snotty comments and all the posters saying they could whip this up in 10 minutes for a fiver.
It's a lovely cake and you have clearly put a lot of time and effort in. I would be delighted if a family member made this for me!
In terms of making and selling, you're in tricky territory. You're up against the supermarkets who can sell their cakes at a decent price point due to economies of scale and cake makers who have a lot of experience and charge £££. The general public can't differentiate between the two because most people don't go to the effort themselves and have no idea of the cost/time involved.
In terms of pricing, you need to work out what it cost you in terms of ingredients, energy and your time then decide on a figure that you are prepared to sell at (rather than just asking what people would pay). To make the numbers work better, you should be bulk buying ingredients and approach the task like a production line so that you can bake and make cakes two or three at a time. Your cake is lovely but it's not professional standard so I would work on honing your technique otherwise you will be criticised.
Never ever start by offering your product low priced to drum up business not even for friends and family. This is a thorn that will stick in your side if the business grows as people will be offended when you put up your prices to what they really should be. Instead offer 10% friends and family discount to your nearest and dearest.
In your shoes, I would contact a local care home or day centre, explain your plan and see if you can offer your time for free but using their ingredients and/or kitchen/oven. This way you could start with a couple of generic Birthday cakes and get lots of practise in. You'll improve your technique with the practise and have lots of photos for social media. Clearly, you'd need to time limit this as you don't want to work for free forever but it might give you a good starting point and contact for future business.
For this type of business, I would say you need to practise, practise, practise, get your speed up and reduce your costs to as low as they can potentially go. People will pay for bespoke cakes but you need to be really good.