Hello javv5 and welcome.
There are only 4 exceptions to the law which says that PCNs for PARKING cannot be issued using cctv to capture the offence and then issue you with a ticket by post. They are:
1. Parking on a red route
2. Parking in a bus lane
3. Parking on zig zag yellow lines outside a school
4. Parking in a bus stop
Unfortunately for you number 4 applies, and unless anyone on here can think of another way to contest this ticket, you might want to consider paying up.
However, for future reference (and you might want to tell your mates about this too), if you had stopped on any single or double yellow for 10 seconds to let your mates out, the only way they could issue a ticket would be if a traffic warden (civil enforcement officer) was standing right next to where you stopped and tried to slap a ticket on your windscreen.
Consequently, if you had stopped on any of the double yellows behind, in front of, or opposite the bus stop, you would have got away with it provided there was no traffic warden hanging round ready to pounce.