The Militia were a very old home defence unit, dating back to
Anglo-Saxon times. They were part-time soldiers, selected at random
from each parish, who trained in their spare time and were 'called
out' in time of civil unrest or invasion. The Worcester Militia helped
to track down the Gunpowder Plotters in 1605, when Guy Fawkes tried
to blow up Parliament, and served throughout the English Civil War
in the 1640's.
This uniform dates from the 1830's, after the Napoleonic Wars when
the Militia had been on almost permanent duty. The hat (shako) and
plume are so high and wide because they are supposed to make the wearer
look taller and more impressive.