For one day I really wouldn't stray from LT. Just smilingly tell colleagues/clients its a special diet, or "medical advice, but nothing serious" and don't eat and just drink sparkling water. they are unlikely to ask anything further, i would imagine anyway - most people are very discreet when it comes to medical matters.
you can still have a fab time, but also be clear headed
i've done weddings, birthdays, weekends away, music festivals, family stuff, hen weekends - all sorts - on LT and had a great time at them all.
it would likely affect weight loss for 2 weigh ins, so it just comes down to whether you are prepared to make that sacrifice for one single event. and whether you think you can get back on LT 100% straight away after a week of having food. (i only say that cos i don't think i would have been able to so early on!!)
good luck with your decision and have a fab day out, either way!