There's a market in London that has a food stall that should serve traditional Lao dishes like Larb and Tam Som (or more specifically Tam "Mak Hoong" if you're specifically referring to papaya salad). If you don't care whether the Lao salad is made from papayas, cucumbers, carrots, mangos or anything else, then it's okay to use the generic term "Tam Som". Som just means "sour". But if you want papaya salad, then for clarity you should ask a Lao person for Tam Mak Hoong rather than use the generic Tam Som term.
Here's a photo of the food stall in London:
For more information about the market, here's the link to the site:
Based on the information from that site, I believe that London market is called the Sunday UpMarket and again one of the food stalls there serves Lao cuisine.