Well it's sufficiently high to be making a contrail !
Lots of traffic at various levels routes overhead Heathrow. In the last year I've been on Paris-Birmingham at cruise level, and Cork-Stansted at a lower level. Come down on a clear contrailing morning and look at all the high level traffic heading overhead.
Just because there's one contrail visible doesn't mean it's single engine. Twin (or triple) rear engined aircraft will give this effect as the engines are close together. Only where engines are wing-mounted are they sufficiently far apart to be distinguishable from the ground.