I can't see IAG bringing the Level brand to LGW there is absolutely no benefit for them in doing so, with British Airways having the B777 going 10 abreast to increase seat capacity to compete with Norwegian is already underway and the Level brand will only dilute and confuse the situation at LGW.
Level will be more of a European focused brand with bases in BCN, FCO, ORY, etc and if the brand is ever brought to the United Kingdom I think that we're see Level at MAN long before we're see them at LGW.
With all the concerns about IAG having to compete with Norwegian well that airline might not even be around long enough to be of any long term concern for IAG.