What is the average capacity of stadiums in the City of London that belong to schools with an enrollment less than 30,000?
CREATE TABLE "ontario_university_athletics_yates_cup" (
  "institution" text,
  "team" text,
  "city" text,
  "province" text,
  "first_season" real,
  "head_coach" text,
  "enrollment" real,
  "football_stadium" text,
  "capacity" real
);
SELECT AVG("capacity") FROM "ontario_university_athletics_yates_cup" WHERE "city"='london' AND "enrollment"<'30,000';