Kolibear,
You mustn't fly in the meantime and you must must call your AME, or better still the AMC at Gatwick and inform them. According to the JARs you must tell them as soon as is practical about any invasive surgical procedure; Appendictomy certainly counts as one of these (JAR-FCL 3.040 refers).
They will then suspend your medical until such time that you are pain free after the operation. They will possibly require a report from the surgeon who carried out the operation as to the prognosis.
I went through roughly similar surgery last year, my medical was suspended for about five weeks, and they needed a short letter from the surgeon.
(Circuit basher's experience sounds unusual - maybe that was how things were done under the UK regs - or maybe this AME was less strict than the AMC would be so it's worth contacting your AME first?)
Get well soon!
foggy.