Google "lengths of British Mountain ranges", the Wiki page has a worldwide list, look for Europe then look for the countries making up the UK - several are listed.
lol even wiki calls them a 'low rising mountain range' even though they don't qualify geologically as mountains .
http://en.wikipedia.org/wiki/Pennines
from wiki:
The mountains are not very high and are often referred to as fells. The highest is Cross Fell in eastern Cumbria, at 2,930 feet (893 m) while other principal peaks include Mickle Fell 2,585 ft (788 m), Whernside 2,415 ft (736 m), Ingleborough 2,372 ft (723 m), High Seat 2,328 ft (710 m) and Wild Boar Fell 2,324 ft (708 m), both in Mallerstang, Pen-y-ghent 2,274 ft (693 m), and Kinder Scout 2,087 ft (636 m).
(Mountains should ideally by 3000ft asl to qulaify as mountains)