EI-BUD, I think you hit the nail with your comment about FR being a deterrent. Norwegian moved away from STN partly to distance themselves from FR (DY isn't such a low sevice airline as FR is), but there were other reasons like different socio-economic profile (10% higher share of A/B at LGW), larger base of potential passengers with UK citizenship (22 million vs. 10 million), and we may also add the differences between the two regarding public transport in to London.