From the Mountain Rescue Council of England and Wales :

British mountains can be killers if proper care is not taken. The following notes cover the minimum precautions if you want to avoid getting hurt or lost, and so inconveniencing or endangering others as well as yourselves.

Clothing
This should be colourful, warm, windproof and waterproof. Wear suitable boots with a treaded sole, not shoes or trainers. Take spare warm clothing and perhaps a hat and gloves; it is always colder on the tops.

Food
In addition to the usual sandwiches, take chocolate, dates, mintcake or similar sweet things, which restore energy quickly. If you don't need them yourself, someone else may. Becks on fells are drinkable if fast-running over stony beds.

Equipment
This must include map, compass, and at least one reliable watch in the party. A whistle, torch and spare batteries and bulbs (six blasts or flashes repeated at minute intervals signal an emergency), and. in winter conditions, an ice-axe and survival bag are essential. Climbers are all urged to wear helmets, especially in winter conditions.

Company
If in groups, make sure party leaders are experienced; do not leave on or two behind to rest and catch up later. Take special care of the youngest and weakest in dangerous places. If you prefer to go alone, be very careful at all times. Let people know your route at start, and stick to it as far as you can.

Emergencies
Don't press on if conditions are against you - turn back even if it upsets your plan. Learn first aid, and keep injured or exhausted people warm until help reaches you. Get a message to the Police for help as soon as possible, and report changes of route or time-table to them if someone is expecting you. The Police will do the rest.

Dangers which can be avoided  all should be until you know how to cope with them:

Precipices, Slopes of ice or steep snow ,  Very steep grass (especially frozen) ,
Unstable boulders , Gullies and stream beds , Streams in spate , Snow cornices on ridges or gully tops ,  Over-ambition , Plain damned carelessness

Dangers which may surprise you  and should be guarded against:

Weather changes - mist, gale, rain or snow  (get forecasts, and watch the sky in all quarters)   Ice on path  (carry an ice-axe and crampons - and know how to use them)
Excessive cold or heat  (dress sensibly, and take s spare jersey),  Incipient exhaustion  (know the signs; rest and keep warm) ,  Accident or illness  (don't panic - if you send for help, make sure that the rescuers know exactly where to come), Flight of time  (learn your own pace - plan your walk - allow double time in winter conditions)

It is no disgrace
to turn back if you are not certain. A party must be governed by the capabilities of the weakest member.

Links

Mountain Rescue Council of England and Wales           -          

Lake District Mountain Rescue Association       -          

Search and Rescue Dogs Association    -          

Duddon and Furness Mountain Rescue Team    -