"Most of the money they are lent by savers they lend out again to people who need it for some purpose"
In fact they don't just lend out 'most of the money' they are lent by savers... they lend out many times *more* money than they are lent by savers
the amount of actual savings on the books versus the larger amount lent out is the reserve ratio
there is no minimum reserve ratio in UK (expressed as percentage it's typically low single digits)
http://en.wikipedia.org/wiki/Reserve_requirement#United_Kingdom