Connecting flights: Is this enough time to go into London?

Hi.
My flight should arrive in London Heathrow at 7:20 AM on a Sunday. My connecting flight leaves Heathrow at 3:35 PM that day. My connecting flight is with El Al, which requires you to check-in 3 hours early. I am very tempted to take a super-quick trip into London during the wait. I have been there before and I know my way around. Do you think that it is worth it? Too risky for getting connecting flight? Thanks.