72 Hours Visa-I am travelling on my Airline Crew standby ticket but will only be staying 72 hours.

Hi!
I am BA Airline Crew and am travelling as a tourist with my partner who will be the BA pilot on both sectors to CTU from LHR and then back to LHR 72 hours later. I will not have a return ticket until I fly back from Chengdu as I will be on standby. Will this be a problem?

My partner will be the pilot flying back so I will have no problem going back to London. Would it be safer to obtain a tourist L type?

Regards
Mel