If you are quoted £5.00 per head for a small to medium party or wedding, in order to make the event viable the caterer will probably be using white fish (the flesh of many different types of fish) rather than pure fillets of cod or other fish.
It is important to bear in mind that the average price of fish and chips in a chip shop in the UK is around £6.50, this is based on you travelling to the shop, queuing and collecting your food.
If you are hiring a fish and chip caterer you are not only paying for the food but for the fuel to and from the venue, travelling time, gas bottles, petrol for generator, generator hire, exclusive hire of the catering unit, wear and tear, staff wages etc
Although you may only want your food served over a 1-2 hour period, the caterer may have a 2 hour journey each way, need another 2 hours to set up and 2 hours to clean down and pack away at the end of the event. So a 1 hour serving period can be a 9 hour day for a caterer.
Therefore, if someone is quoting you a price that seems too good to be true, then it probably is and you may be let down at the last minute!