FlyerTalk Evangelist and Ambassador: The British Airways Club
Join Date: Jun 2008
Location: Somewhere between 0 and 13,000 metres high
Programs: AF/KL Life Plat, BA GGL+GfL, ALL Diam, Hilton Diam, Marriott Gold, blablablah, etc
Posts: 33,183
Are you planning to take any trip to Canada/the US between now and July? Many fares are combinable on a half fare basis with other airports in the same area (ie North America), so you might be able to change for a YVR-LHR//LHR-JFK for instance (as Anonba says, if the ticket is changeable). That would still leave you to find another way to return from JFK (or anywhere else) to London.
Your idea of using value for a brand new ticket is quite certainly a non starter though.