How long is a piece of string?
The Gardeners' Guild (which obviously wants to tell you how much you should value a good gardener's services) says "Most traders aim to earn a minimum of £150 a day - that's £20 - £40 per hour":
https://www.thegardenersguild.co.uk/how_much_should_you_expect_to_pay_a_gardener.html
Bidvine gives £14 per hour as a minimum and £25 ph as the maximum, with an average of £20 ph:
https://www.bidvine.com/gardening-services/price-guide
MyJobQuote says "Garden maintenance prices can often be difficult to determine, but most gardeners work off an hourly rate which can vary from as little as £15 per hour up to £50 per hour":
https://www.myjobquote.co.uk/costs/hiring-a-gardener
I've lost touch with my mate who did gardening work (and he's retired anyway) but around a decade ago he wouldn't touch a job for less than £12 per hour.
There are a couple of guys with a chainsaw who cut back my hedge (so that it'se not blocking the public footpath) from time to time and who then take all the cuttings away. It's usually only about 15 minutes work for them in total, for which I typically pay about £60. (I reckon it's worth it, as it saves me several trips to the tip to get rid of all the cuttings, plus the hassle of getting them all into my car). So I'm effectively paying them each £120 per hour for the time that they're at my house (although, of course, they'll spend additional time getting rid of the cuttings).