Not completing flight.

Your return ticket will be cancelled if you do this without permission from the airline. Best to call and ask if this is possible.

DJ

If you don't fly on to Paris your booking for your flight home will be canceled. If you want to read more about this issue, search for "hidden city ticketing."

Your return ticket will be canceled if you do this. Either pay the change fee or fly onto Paris and return to London by Eurostar or a regional flight.

You are flying to Paris with a stop over in London. You would like to stay n London and THEN continue on to Paris. As far as I know, you MUST adhere with the schedule of your ticket.

The way to do London then Paris and presumably returning to your home city would be to book a multi-city ticket. If you already have a home to Paris via London, you need to fly the route or risk your ticket canceled if you miss a segment.

See if the airline agent can extend you layover in London. I don't know what is the maximum length for a layover but I have search for tickets that have almost a 24 hour layover, Otherwise it can't be done without voiding your return ticket. Why don't you fly to London and take the chunnel to Paris?

It's called "hidden city ticketing". One writer's thoughts:

http://www.huffingtonpost.com/barry-choi/why-hidden-city-ticketing_b_7883384.html

Partial "reprint" below:

How Throwaway ticketing works
For those unfamiliar with Skiplagged and hidden city ticketing, this is how it works.
You're looking for a flight from A to B. Let's say Chicago to Charlotte
Skiplagged will find you a one-way ticket from A to C with a stopover in B. Let's pretend there's a Chicago - Charlotte - Atlanta route. This route might be cheaper than buying a direct ticket for A to B
Book the A to C (Chicago to Atlanta) route, and depart the plane in Charlotte
Throwaway ticketing has been a lesser known travel hack for years, but sites like Skiplagged has made it easy for the everyday traveler to take advantage of. There's potential to save money, but it's not completely risk free.

Negatives

No checked bags are allowed
Cannot book round trips. If you miss any part of your trip, the rest of the ticket is cancelled
If the stopover destination changes, there's nothing you can do about it
Violates rules of the carrier
What happens if the flight gets rerouted and the plane no longer lands in the destination you wanted? You're out of luck, the airline only needs to get you to the final destination on your ticket. So think twice before using this hack just to save money.