You could if you were so inclined to, phone the dealer and arrange a test drive. Then after the test drive say that you do not like the car and directly ask for your deposit back. If they then refuse you should go online and issue a moneyclaim. The online fee for amounts up to £300 is only £25. That does not mean they would cave in and give you your £100 back but a CCJ recorded against them could cause them problems and for me that would be satisfying.
On a separate note, this is a problem that has cropped up on this forum many times. I am always surprised at how people are prepared to take someone’s word when involved in a financial transaction. Always best to use email and then if any promises are made at least you have them in print which strengthens your position if and when things go wrong.