A bit more has occurred to me, although I'm sure the original poster has already solved or cancelled this problem...
Remembering from my time in small electronics repair:
1. 90% of the stuff we get in the USA was originally designed overseas. Most of the world uses 230/240, so everything is designed for that and then modified to work here. Sometimes the modification consists of specifying a completely different transformer for the US version. Often, however, the power supply transformer simply has different primary taps for different supply voltages. It may be possible to turn the US version of the oven into the UK version simply by moving a jumper wire on the board.
2. Regarding line frequency: The most popular clock/timer IC (don't know a specific number off-hand, as there are like 30 billion different versions of it, mostly pin-compatible, and all function-compatible.) has a logic pin that selects 50/60Hz. I'm sure this exact IC isn't used in the oven (as the oven needs other functions said chip wouldn't provide, requiring a seperate µC, and it makes much more sense for the clock to be integrated in the firmware), but there still may be similar.
