Yeah .. VIVA Aerobus is fine ( despite the odd name ) .. it's only a short flight .. I think there's Volaris as well, but I've never managed to make their website work to buy a ticket from the UK
AeroMexico is 'nicer' but more hassle transferring at MEX T2>T1 on the train so unless you're desperate for SkyTeam points or insist on flying Business I wouldn't bother really