A lot depends on who you are employing, what their level of experience and competence is and what qualifications they have - I would put experience and competence way above qualifications as a prerequisite.
Someone fresh out of horticulture college may be a brilliant academic and they may know their Latin plant names but it's often the case that they lack practical skills.
As far as money is concerned.
An odd job gardener - i.e. one who may be really competent at cleaning up the garden and weeding but not necessarily knowing plant names and being able to truly identify a weed from a desired plant. An odd jobber will be happy to paint sheds or clean patios too and maybe clear gutters of leaves.
The price they charge will depend their needs but you should not expect to pay below £8.00 (assuming that is they are legal and not doing a black market job).
Choose a person that you like - this is absolutely essential.
Next, ascertain what level of experience and competence they have and if you should be looking to pay a minimum of £15.00 per hour but not in excess of £18.00 per hour.
A fair market rate for a one man band who is running a single van with just hand tools and a mower would be £15-18.00 - but expect to pay extra for machinery and materials used.
Employing a contracting firm - a contractor might be an established firm with a small fleet of vans, with 3-20 men on the payroll and the skills and resources to scale up onto a variety of jobs. Expect to pay £20-£50.00 per hour - depending on the level of skill required.
The larger firms are best suited to large scale private residential or commercial activities and employ a range of labour skills from basic labourer through to skilled operatives.
In large cities such as London, market forces might dictate that you pay more for gardening and landscaping services.
If you are looking for high quality service on a one-to-one basis then employing a one-man-band or even husband and wife team can yield the best results - but be prepared the pay the right price;haggling on the price may be reflected by the quality of the workmanship and/or how productive you gardener may be.
Do not, whatever you do, sign up for a long term contract until you are sure of what you are getting.
Do you run a gardening service? Discuss 'what does a gardener charge' here
Further reading - What does landscaping cost