I applied through priority service on 12 Dec 2018 from Lahore, documents sent 18 Dec, application ready for ECO email on 8 Jan 2019, visa stamped 16 Jan 2019, called to collect on 21 Jan, visa valid from 25 Jan. I had previous refusals and overstay and considering xmas and new year holidays, it was really quick. I would definitely recommend priority service. I have seen few other members applying through priority after me.
When you go for biometrics, they always ask you if you wanted any extra services including priority, SMS notification, etc. I paid for priority service online but paid for sms service in the centre. This is why I was saying you should call all the centres to see if they have service available. Ask her to take bank card or cash (preferred) with her, just in case the service is available in the centre.