The route into Consultancy in arb can be varied but the starting point is a Level 3 in Arboriculture. If you don't have a Level 3 and have plenty of industry knowledge then you may be able to choose to move straight to the Level 4 with an online provider; for one example click here but there are lots of other Level 3 and 4 courses you can read about online.
If you already have a Level 3 or above, great, the next thing to think about is your experience.
Consultancy firms looking to recruit always ask us to find arborists who have working experience and a good understanding of BS5837 (trees in relation to design, demolition and construction). We can't stress how often this comes up or how important it seems to be so if you can evidence this you're ahead of the pack. If you can't then read up and start applying it where possible in your current role so that you become confident with it ahead of time.
They also often ask about CAD and will expect experience and confidence with handheld GIS mapping devices. Additionally you'll need to be an excellent communicator who is comfortable with computers and the written word.