Don't forget money for insurance......lots of it.
You will have to buy any machinery for your shop, depending on what the scope of work will be. Tire mounting/balancing machines, car lifts, compressors, mills and lathe, welders, presses, diagnostic equipment ( anyone know where I can get a Vag-com?lol) and on and on.
Then you have to get approval by the government, here in the land of hockey and COLD beer you need at least 50% licenced staff.
Then you need to set up accounts with parts suppliers, so a good credit rating is a must.
I understand you have manditory e-testing in the U.K., so you will need that equipment, government certification and trained employees.
I have people ask me all the time why I don't open my own shop (I am a licenced m/c mechanic) to which my answer is " I want a life" A good friend of mine has a bike shop and that is his life. It has wrecked friendships, relationships, credit and reputation along the way, but he is still plodding along. I am much happier and more importantly have more money and time to myself as a result.
But hey, good luck, there's no saying you won't do well. I would suggest you get a job in a successful shop and learn how they do things and what they have/use to get a baseline for what you should expect.