I had this last year, it cost about 90 pounds. This means that if one of your guests drops dead from poisoning, falls over drunk and splits open their head on your marble table, you have insurance to cover it.
You can also get equipment insurance, legal cover and ....here's the sticky bit: cover for your staff.
Your 'staff' whether volunteers or paid employees need cover if they drop a hot bowl of soup on a guest, if they injure themselves.
This becomes quite expensive. I just got a quote for basic public liability which covers me, for £79, add on equipment value £10k then it increases to almost £300, add on legal insurance, another £30.
However once you add 'staff' things become very expensive ...£600p.a. or more depending on how many you have.
I haven't bought this staff add on yet. But let's face it, it's probably a neccessity.
If you are truly underground then I guess you don't have to bother. Unfortunately (or fortunately) I am now so 'overground' that I probably do.