Why?
All everybody is advising is that she play it by the rules, as tempting as it is to "stick it to the big companies - they can afford it!".
Your comment might hold water if it was just a single agency (Tesco) for £17, but it's not.
One wrong move here and you potentially have multiple agencies on your back which can have much more serious implications in future.
Don't declare Epilepsy to DVLA - Mother's licence technically becomes invalid...
Mother's licence invalid means 2nd Named Driver is not legally permitted to drive, contrary to what was declared on the insurance application form, resulting in insurance policy potentially being deemed invalid as OP has lied to her insurance company...
OP then drives without realising the insurance has been cancelled - risks getting prosecuted for driving with no insurance - 6 points, a hefty fine & the car impounded.
Meanwhile, the insurance company logs the OP's "mistruths" on the central Insurance Fraud Database which all UK insurers have access to...
OP Struggles then to get any form of insurance (Car, Home, Life, etc.) in future, or at massively inflated prices, due to the black marks on the Insurance Fraud database.
Just so she could save £20 per year on her car insurance...