I'm with KBIS and you can choose your level of cover - they cover up to £7500 for colic surgery. Unless, like me, your horse is 16 or over and classed as a veteran :-\
A slightly complicated colic op with follow up could easily be £8k so I've always thought a £5k limit was on the mean side.
I had an emergancy joint flush and few days at horpital for IV anti biotics, all very simple and that was £3.5k without blinking!!