Sorry to break the bad news but I think the dealer is correct in his assumption. Open the bonnet and listen around the drivers side windscreen wiper area while an assistant turns on the ignition (without starting the engine). On a normal car, you should hear the motor in the ABS servo run for about 20-30 seconds to pressurise the VSC/ABS module. If the motor runs continually then the required pressure is not being attained and, it's probably this that triggers the lights and warning sound. A dealer once told me that you can replicate this by continuously stomping on the brake pedal (with the car stationary) as this also causes the motor to run longer than it should. Maybe a scrap dealer would be able to supply one at a cheaper price, or maybe someone else in the club knows of an ABS specialist that could refurb yours. I take it the dealer checked the obvious things like brake fluid level and whether any caliper pistons were leaking?
I'm almost getting to the same stage as you. I've had nothing but problems since I bought the car. OK nothing this serious, but I feel like all the small problems are mounting up and I can't help thinking what's going to be next. At least I managed to end up with a 3 year warranty when I bought mine. The dealer gave me an extra year to try and make amends for the problems, and Lexus GB also gave me an extra year because of their apparent inability to spell, get dates correct and identify the correct model of their cars on my warranty certificate (do Lexus even do a 6 month warranty?)
Good luck getting it sorted and I hope it doesn't cost too many $£$£ as Christmas is coming up fast!!