Most professional gardeners make between £20-30 per hour. I've heard of gardeners in Knightsbridge charging £45 but that's the exception rather than the rule.  Experience and knowledge do not cause drastic changes to this compared to other professions.  I've spoken to gardeners who work at botanic gardens, stately homes as well as those who do small gardening rounds for domestic customers.  

A head gardener who manages a team often has over 10 years experience gets paid marginally more than somebody operating operating a lawnmower.  These head gardeners are, on the whole, people who have chosen their career through their passion not for their salary.  If you can get somebody like this, hold onto them like glue!

The average wage as a gardener according to Total Jobs is £20,536. Assuming one can work 40 hours a week for 48 weeks of the year that equates to £10.70 per hour.  This is also the advertised rate for pay through agencies at the time of writing.  This figure is corroborated by the fact that self employed gardeners charging £20 would be at around this figure when expenses and deductions have been considered.

One answer to this is that many gardeners charge a fixed price and invest in machinery to help reduce their time on site.  This sometimes results in a higher margin per job. The only thing to bear in mind is the time taken to go and quote every single job.  The jobs that aren't won also have a cost.  I've worked out on my own jobs that it takes around 4 hours total quoting to win a job, considering the jobs that fail to get accepted.  That means that If I do a job charging £200 for 6 hours work, I didn't make £200 / 6 hours (£33.33 per hour) I made £200 / 10 hours and therefore £20 per hour.  

As a customer, it's far more important to consider how well you trust the contractor.  Do you have a recommendation from another customer?  Do you feel like you can trust them?  Will they work as productively as they can?  These are far more important considerations than the rate they charge.  They will ultimately make a bigger impact on your overall satisfaction.