It sounds pretty thoughtless to me - especially if you are a working parent and/or need the after school club to keep your life together. Especially if it's on short notice.
Our school always gives the option of pick-up from venue, or pick up (sometimes later than usual) from the school.
Do you have a PTA? Could they represent parents who are in a fix about this? As the school is presumably taking the children there, the PTA could, on behalf of all parents in need, ask for those uncollected to be taken back to school and looked after until the usual end time of the after-school club.
The school are being very unreasonable. You need to go to them and say that you will be picking up your DC from school, what time will they be back? Make it clear that you have no transport to get them. I don't think five miles is a reasonable distance to expect a child to walk after sports day.
So far it has been OK as I have been off work when I have needed to pick her up. On the days I work she is collected by an after-school club. They will only pick her up at school as they have other children they need to collect and they are unable to collect her earlier or later than usual school finishing time. If the school were unable to return her school and I couldn't find another mother to help she would have to stay a school and miss her trip.
I think they think that all mothers don't work or have loads of family to help out.
(Are you sure there wasn't an earlier notice you missed? Not that that helps now).
As it coincides with strike day, perhaps they've had difficulty in ensuring they had enough staff to run the event safely? A possible reason for late communication (but a poor way to foster parental support)? But as venue/coaches must have been booked in advance, it doesn't explain the one-sided arrangements.