There are opportunities for people with a very wide range of interests and training in horticulture, thanks to the large number of sectors into which the head can be divided. A shortage of skills means that anyone with the ability and the inclination is going to have a happy time, although there are professional and academic qualifications, without which entry and progress may be slower.
As the Institute of Horticulture Jobs says: "Employers need staff who can understand legal requirements, prepare costings, operate computer systems and manage other people. With an increasing number of horticultural jobs being linked to public contracts, it is vital for firms to demonstrate that their staff are up to the job - and the best way to do this is by being able to show formal qualifications."
Setting up on one's own is going to be attractive to many, but first you may need to learn your trade. Examples of job areas within horticulture include: Garden and landscape design; jobs in historic or botanic gardens; Production horticulture jobs - edibles; Production horticulture jobs - ornamentals; Sports turf care jobs/groundsmanship; Parks and amenity horticulture jobs; Forestry, Arborists and arboriculture jobs; Garden retail jobs; Contracting and grounds maintenance...
Salaries are not great so you need to really enjoy what you do. One way to gain experience is by volunteering first - this is a recognised way in and gives you a feel for what it is like. Being a professional gardener is not the same as doing your garden at home, and some career changers find that it was not what they expected. But if you enjoy it, it can a brilliant and fulfilling lifestyle.
Other good ways in are the traineeships offered by a number of gardens (Hilliers, RHS, Oxford Botanics, Birmingham Botanics, Edinburgh etc.). The Kew diploma and the National Trust also are well respected. Alternatively, many go to college and do a practical qualification.
The Historic and Botanic Garden Bursary Scheme has opportunities for training places.
'Grow' is a horticulture careers website supported by a collection of the leading organisations in UK Horticulture: www.growcareers.info, where you can find course listings, career profiles and links to the Horticulture Jobs website. Horticulture Week has a large career guidance section and is a good place to get a feel for salaries & jobs, as well as information on short courses and training and qualifications: www.hortweek.com/horticulture-careers The main organisations also provide their own careers information: the RHS, the National Trust (www.ntjobs.org.uk), etc.
Specialists in forestry / arboriculture would refer to the Arboriculture Association (www.trees.org.uk), Forestry Commission (www.forestry.gov.uk) and the Royal Forestry Society (www.rfs.org.uk), all of whom provide good careers information on this side of things.
Career progression can be slow; the industry can be poor at recognising the existing skills of career changers so it is likely you will need to start at the bottom. Look at the options out there and talk to some people in jobs you might aspire to before taking the leap so that expectations are realistic.
That said, working in a beautiful garden on a sunny autumn day doing practical, skilled work can be the best feeling in the world!