I have a Zojirushi 1295w rice cooker from Japan. Other than buying a 240v to 100v transformer Is it possible to modify the cooker itself?

No.

Just buy a UK standard one from Argos
http://www.argos.co.uk/static/Product/partNumber/9367809.htm

It may be possible, however it may not be probable unless you know what you are dealing with and have the proper knowledge.
I was able to modify a slow cooker to work on 240 volts from 120 volts with the use of a diode and re-arranging the switch contacts to put the elements in series instead of parallel.
This however is not something that a novice can attempt!
A properly sized transformer will cost much more than buying a new unit that is designed for 240 volt operation.
Although that is sad it is true.
Good luck.

Possible. Yes. Easy not without really understanding electricity. And realizing that ur cooker is gonna take twice the current it normally would