It's absolutely supply and demand and Gatwick is getting interest for expansion and those that can no longer retain the LHR slots they need, in the case of ITA for example who were leasing slots.
Personally I think the CAA pricing caps at LHR are counterproductive to the overall LON airport system. I get why they're there principally to protect the public from price hikes due to LHR's dominance in the market. However, it shows that airlines/routes that don't need to be at LHR from an onward connectivity/shared alliance point of view are still willing to pay for it's prestigiousness or still achieve the yields that set LHR apart from the other airports. The reality is many of these foreign airlines are at least partly government owned and those countries will want their national flag carrier to fly into LHR.
Take the Air India routes besides Delhi and Mumbai for example, or someone like Biman Bangladesh, who are very point-to-point, VFR market focussed. They can easily serve that demand from LGW or STN. LHR offers those routes/passengers nothing other than the fact that it's Heathrow.
If more of those routes, including the uplift of African services for instance are now being drawn to LGW rather than LHR then that's a positive development in my view. It would better allow LHR slots to be used more effectively by those airlines and routes where LHR is strategically important, namely hub connectivity, rather than simply the fact that it's Heathrow and looks good from a prestigiouness/perception point of view.