Having done the LHR FCC 5X this summer, and having the shortest time gate-to-gate (including the UGS minibus) been 60+ minutes, you're more or less counting on the LH flight to be late. These days the line for the FCC is typically broken into 3 sections:

Section one, down stairs, essentially below the screening area itself. In this section order is strictly maintained and the line inches forward at a barely perceptible rate. Expect 15 minutes here.

Section two, up stairs, across the bridge from the screening area itself. In this room pax (from other terminals I assume) join the chaos and any illusion of order breaks down. Expect 10 - 30 minutes here depending on how willing you are to trample 8 year olds (you will succumb, it's just a question of when) to get to stage 3.

Section three, the actual screening queue is initially chaotic as BA pax's split off to their own "special" queues apart from the rest of we unwashed masses. Once the mad jostling is over things move fairly well. Expect no more than one magnetometer to be operating and no more than 2 x-ray's. There are plenty more, but BAA apparently likes to keep their equipment in pristine, like-new condition. Expect 20 - 25 minutes in stage 3.

Short version: You're gonna miss your LH flight.