(I have to be aware of the alternatives because I don't have a smartphone. It's not been an insurmountable problem yet.)
Some have extended to non-residential areas. One of the biggest parking apps says this:
Q: "I have no network coverage and need to park?"
A: "Move quickly to an area with better network coverage and complete your parking transaction with PayByPhone... Pay for your parking via other methods such as PayPoint or via the Pay & Display machine where available"
Reference: https://support.paybyphone.co.uk/hc/en-gb/articles/360010478...
FYI 'Pay and Display' are the parking meters being removed, 'PayPoint' might be in SOME convenience stores/shops. But if you know London these can be 20-30 mins walk or none there are all at your parking destination. Good luck attending an appointment during work lunch times!
Tangentially - I’m always astonished at the lack of self-awareness of many municipal/utility service providers - they pretend like I’ve sought out their service and WANT to engage with them, when in fact I just want them to provide me the thing I need (or am obligated to do) with as little hassle as possible…