"If it flies directly" means that it will fly a great circle route.

Here's the route (London is at the bottom of the image, Los Angeles at the top):

Image from Google Earth.

We learned above that the latitude and longitude for London and Los Angeles are as follows:

London: φ₁ = 51°32' N = 0.899426,   λ₁ = 0° 5' W = −0.001454

Los Angeles: φ₂ = 34° 3' N = 0.594285,   λ₂ = 118° 14' W = −2.063561

Applying the formula for the central angle gives:

Central angle =

= 1.374296

So the distance between London and Los Angeles is:

s = rθ = 6371 × 1.374296 = 8756 km