I live in an area where tons of data is missing. I mean the whole areas with no street addresses, missing streets, missing bike routes, etc.
Is there a guide or special tools for preparing to make massive edits like this? Is there a particular work flow that people use or resources? How do I even get started?
Then ask the neighbouring group if there's not even one on the country level. It's really best if you talk to those people who map in the area or next to it.
With the project starting in London the mailing lists have a lot of inertia behind them for British mappers. If you join I'd recommend a dedicated email address.
There's a full listing on the wiki, but talk-gb is probably the biggest.