Are you sure the traffic you are seeing is not London City outbound traffic. Most departures turn to the north soon after departure.
The fact that London City is the fastest growing airport in the south east (I think), is probably why you have been seeing this more and more often.
Heathrow traffic on final approach is not normally vectored this far out as it is quite close to the edge of our airspace. Occasionally it is, though we normally aim for a final between 10 and 15 miles.
If the Heathrow final approach is extending this far east, the traffic you see will normally be between 6000 and 4000 feet (never normally below 4000ft).
I certainly haven't seen a trend for planes to be lower in this area over the years. My bet is that you are seeing London City aeroplanes.
AI