Shell
Unless they need the Class 1 Credits that ESA pays instead of UC's Class 3 ones, or they have significant Deductions from their UC I can see no advantage in claiming NS ESA in parallel to UC, the ESA will be deducted £ for £ from the UC leaving no monetary advantage for having claimed ESA.
There are no Premiums payable with NS ESA which would change the equation.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems