Because you have to declare If you have EVER used the NHS - if it turns out she needed to pay and didn't and the outstanding unpaid amount was more than £1,000, it's an automatic visa refusal, so they have to check.
MelFromLou - if you can remember anything about where you went (town, city, county), give as much information as you can and explain it in the additional info and/or in your cover letter - if it was just a basic appointment and prescription it wouldn't be £1,000 worth of treatment anyway, even if you did have to pay for it.
If the form won't let you move on without writing in the box - put something (I.e. all zeros for the phone number) and then cross it out on the printed version and explain the issue.
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