Norwegian have already announced that they'll move the entire current STN operation (except for STN-TRD) over to Gatwick as of end of March.
Rumour has it that Air Moldova is eyeing up a move to Gatwick too.
Not sure about Air Asia, I may be wrong, but I thought there was an issue in the UK-MY bilateral which meant that there was no more 'space' within the current frequency limit for a Malaysian carrier to operate any more frequencies from LHR or LGW to KUL.