Hi @crispy2019 careers adviser and mum of med student here. My advice to your dd is don't panic and right now take a break from thinking about all of this just for a little while. You've had a shock, you're adjusting to new unexpected future plans and so don't make any rush decisions today about resits or alternative routes to medicine.
When you are ready, take a very honest look at why your A level grades were lower than predicted before deciding if retakes would be a sensible or positive route. Talk to teachers when school is back. There could be a whole number of factors.
Did your teachers over-predict to As in order to get you the med school offers but in fact you were always averaging borderline A/Bs?
Did you work as hard as you honestly needed to all year and this was the best score you could get on the day?
Are you a clever and high achieving student generally, but exams just aren't your thing and you underperform in test situations?
Or
Were there any mitigating factors/ things going on in your life (underlying health/ mental health issues, school, relationship or family issues etc) that meant your lower performance in these exams was 'out of character'?
If you genuinely did the best you could on this year and there aren't obvious reasons why things went off track for you then I would question if you really want to do retakes. Doing 3 self-study A level retakes on your own can be pretty tough and many/most people don't improve their grades. What will be different next year if you retake?
Also, think through, how do you feel about another year of grinding through A level revision followed by a career that requires regular, very difficult exams for the rest of your life? The great majority of people who get into medical school become successful doctors but my doctor dc have had friends who have struggled to keep up, failed exams and dropped off their medicine courses. (One friend failed his final year exams twice). Others just decided they didn't enjoy medicine enough to accept the constant pressure of exams while also trying to live a life and do an intense job.
I'd suggest you use this unwanted/ unexpected gap year to keep your options open and explore alternatives. Talk to teachers, redo your UCAT and sign up for retakes so if you're still dead set on medicine you can reapply, but also spend time exploring alternatives, look into other careers and other courses you could do at unis you'd like to go to, get a job, travel, have fun.
Good luck whatever you decide to do next - medicine is definitely not a closed door to you but a lot of other doors are also open to you right now.