LGW have to have more than 29 CE pax to get the 4th crew member. So, if there are 29, then you have one crew, the Purser usually, doing the whole service for all 29 pax.
I have to say, I have the upmost admiration for my LGW colleagues, they do an amazing job with a lot less tools and man power.
Hope you have a great time in FCO!