This might be hard if OP is outside the UK. If the client responds to the claim disagreeing (i.e. saying they don't owe money), then the next step will be a court hearing. The amount in questions is likely less than the time+money it would cost OP to go to the UK to attend such a hearing.

"You might have to go to a court hearing if the other person or business (the ‘defendant’) denies owing the money and you disagree with their response."