Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
My dear, if Lady cannot breathe and is bringing up white froth (which could be a hint of fluid in her lungs that is causing her breathing distress), there is not going to be any home treatments for this situation. Therefore, there is no other option but to get her to her vet urgently to help her survive the night. Since we are in the UK, not driving won't completely prevent you from getting her help. You can consider phoning a cab to take you to the vets.
Otherwise, phone your vets and let them know the situation. Some have pet ambulance services, some will know of local pet ambulance services, or even know what taxi companies will carry Lady to them with you. If none of the options are possible, then you can request the vet come out to Lady. Now be aware if she needs oxygen, this will not be ideal since they cannot bring the practice to her (only themselves). But they may be able to collect her and rush her back to the clinic or euthanize her if they find her situation to severe on examination.
Finally, just in case you are fearful that your vet is closed for the evening, I wanted to mention that most veterinary practices here do have contingency plans for emergency care for their patients even when they are not open. Therefore, it is worth ringing the practice. If they aren't open, then they will likely have a message to direct you on how to contact their out of hours service. And if you don't have a vet you can find one local to you, you can check you can check the RCVS Register (HERE) to find your local Vets Now (LINK) who are open all nights/weekends.
Overall, struggling to breathe in any species is an emergency situation and not something that can be managed at home. I know not having a car makes thigns complicated but you can still get her to the vet or the vet to her with the above steps. So, you can help her even without driving and at this moment it sounds like your poor lass needs you to act quickly to help her.
Please take care & the best for poor wee Lady,