Why power is the problem at all
A warm buffet means the food has to stay at temperature between delivery and the last course. The common solution is electric warming units. Those need a circuit that can carry them, and that is exactly what fails in old buildings, barns, marquees and gardens — not because there is no power at all, but because the existing fuse will not carry several appliances.
In practice you do not discover this during setup. You discover it an hour later, when lights, music and kitchen are all running.
The alternative: fuel paste
Chafing dishes with fuel paste work without electricity. One tin burns for two to three hours depending on type and holds the dish at serving temperature. No generator, no extension leads across the yard, no discussion with the caretaker about amps.
It also makes the wet-weather plan simple. If it rains, the setup moves indoors without anyone having to re-plan a power supply.
Where the limit is — plainly
Fuel paste is an open flame. In some listed interiors, on most boats and in certain museums, open flame is prohibited. There this solution is not permitted, and that has to be settled before booking, not on the day.
Second point: keeping food warm is not the same as cooking it. A live station where food is prepared fresh needs a real heat source. For that we always agree the energy supply with the venue specifically.
What the venue actually has to provide
A level, firm standing area. Vehicle access for unloading — and if it goes via stairs, say so beforehand and we plan extra time and one more person. A water point within reach helps but no connection is needed. We take our waste away with us.
That is all. No gas connection, no three-phase power, no kitchen.