UK Hedge Plants: What Should I Consider Before Planting a New Hedge?

I'm planning to learn more about establishing a long-lasting hedge in the UK and would be interested in hearing from people with practical experience.

When choosing hedge plants, what factors do you think are most important before planting? For example:

Soil type and drainage
Sunlight and aspect
Desired hedge height and density
Growth rate
Maintenance and pruning requirements
Resistance to pests and diseases
Value for birds and other wildlife
How quickly the hedge establishes

I'm especially interested in how species such as laurel, privet, hawthorn, beech, hornbeam, holly and yew compare in real gardens. Are there particular varieties you'd recommend for difficult soils or exposed locations?

I'd also be interested to hear whether anyone has replaced an old Leylandii hedge with a different species and how the change worked out over several years.

I've been comparing different species and growing requirements through Everything Plants while researching this, but I'd really value experiences from people who have actually planted and maintained these hedges.

What hedge would you choose if you were starting again today, and why?

For me the wildlife angle settled it fairly quickly - a native mix of hawthorn, blackthorn, hazel and field maple and the birds are going absolutely mad for it by year three. The one thing I'd think hard about upfront is eventual height and how much cutting you actually want to be doing each year, because that shapes most of the other decisions.

I'd go with Joao's recommendation. Privet hosts the caterpillars of a hawk moth (good) but clippings fed to animals can be poisonous (Gran fed clippings to Dad's rabbits with fatal consequences - so bad!)
All parts of the yew are poisonous, except for the red flesh on the berries, so best avoided. They grow huge - mostly found in churchyards  nowadays.
Similarly laurel - except for bay trees, leaves of which used in cooking. Bay trees can be grown in pots, and trimmed to keep small. I put mine out in the garden and it's now about 20 feet tall. Beware the others. . .they give off cyanide according to the literature.