Upvote:1
Airlines hate when you do that. If you do not take both legs of your outward journey, they will cancel the return ticket, re-sell your seat, and not refund your money.
You are probably better off re-writing the ticket with your agent or just to Jeddah as planned then heading straight on to India, which will be about $400.
Upvote:4
You can fly to the UK on your original ticket, but your remaining flights will be cancelled when you don't show up for the London-Jeddah leg.
Your best bet would likely be to get in touch with the airline/travel agent and see if they can convert your ticket into an open jaw: Halifax-India, "surface leg" from India to Jeddah, then back from Jeddah to Halifax. This way all you need to do is buy a one-way from India to Jeddah.
Alternatively, just fly to Jeddah and book a return flight from there to India. This would be straightforward and not necessarily any more expensive once you factor in the change fees; but of course it's a bit of a detour/hassle on the way in.