Setting the foundations correctly
Before you optimise anything, the basics have to be watertight. Google trusts profiles that are consistent and verified.
Name, category and verification
Use your real business name exactly as it appears on your van, invoices and signage. Stuffing it with keywords like "Bob's Plumbing London Emergency Drains" breaks the guidelines and can get you suspended, which is a painful thing to recover from.
Pick the most accurate primary category, for example Plumber, Electrician or Roofing Contractor, then add secondary categories for the other services you genuinely offer. The primary category carries the most weight, so do not waste it on something vague.
Complete verification properly. Google now uses video verification for many trades, where you record a short clip of your premises, tools and signage. Have your van, branded workwear and a utility bill to hand so the process is quick.
Service area, not a fake address
Most trades work from home or a yard and travel to customers. That makes you a service area business. Set your service areas to the boroughs and towns you actually cover, and hide your address if you do not receive customers at your premises. Listing fake addresses across London to appear in more areas is a known way to get suspended.
Be realistic with coverage. A profile claiming to serve all 32 London boroughs looks thin. Listing the eight or ten areas you genuinely reach builds more trust with both Google and customers.