I run my own business (I'm a jeweller so my minimum costs are different but will give this a go).
First, work out what annual costs you will have no matter how much business you get for example -
Liability insurance - £200
Tool replacement/maintenance - £300 (total guess but you'll know better)
Business use car insurance - £1200 (again, total guess but you get the idea)
Petrol for jobs (hard to guess for) - £1500 ???
Website/email provider (if you choose to have a professional email and URL) - £250
Advertising/marketing materials - £1200 (choose your own budget but for this example, I've done £100 a month)
Anything else I didn't think of - ???
So, that's your expenditure no matter what - this example adds up to £4650.
Now... How many hours a week do you plan to spend in someone's garden? You'll be doing loads of hours of work outside of this - like social media, replying to queries, driving etc and you can't charge anyone for those hours; all your money comes from hours in the garden. So, let's say 20 hours a week, and you're going to take 4 weeks holiday a year.
That's 20 hours X 48 weeks = 960 hours spent working in a garden each year.
To earn your costs back £4650 per year/960 hours per year = £4.85 per hour.
Now, you're profit. Let's say you want to earn £24,000 a year -
£24,000/960 hours per year = £25 per hour.
Add £4.85 and £25 = £29.85 per hour.
If you're planning to push for full time, then you'll work more hours and bring that cost down and you might not have as many expenses as I've guessed at but you get the idea. Maybe offer cheap rates for basic work and high rates for fancy work? Someone with more gardening experience might give you a better example but that's how I work mine out.