Normally the questionnaire is for passengers in an adapted cabin, to confirm they do need to be in one. But if your Mum does not use a wheelchair, of her own, then it might be useful to request boarding assistance for which P&O would provide a wheelchair and pusher to help your mum with boarding and disembarkation back in Southampton.
In fact it may be very useful for her if you hire a wheelchair for the duration of the cruise. The preferred P&O partner for this is Mobility at sea.