They can grow outwards as far as the neighbours (who've got the hedges intruding onto their land) are prepared to let them.
However if a neighbour's hedge (or other vegetation) is growing across your land you're entitled to cut it back as far as the boundary without fear of legal redress. (You're supposed to return the cuttings to your neighbour, to avoid being accused of theft, but many people wouldn't bother).
With regard to the height of hedges, you can complain to your local council if a hedge exceeds 2 metres in height and is affecting your enjoyment of your garden or property. However you have to pay a fee for doing so. (If the council then accepts your complaint as valid, they can order that the height of the hedge be reduced).
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/9411/highhedgescomplaining.pdf