There is a good deal for flights from Oakland to London Gatwick on British Airways with ticket prices as low as $457. You can drop the ticket price by $65 with AARP discount for $392 round trip. There are low fares at $502 from SFO and SJC too with American for British Airways operated flights.
If you can find BA tickets, then you can apply AARP discount.
Low fare departure dates are April-May, mid-September to January 2018.
British Airways Oakland to London Gatwick LGW nonstop
American Airlines San Jose SJC – London LHR – Phoenix PHX – SJC
American Airlines San Francisco SFO – London LHR – Phoenix PHX – SFO