If you contracted to buy the car on trader premises there is no cooling off period and therefore the trader does not have to refund the deposit. When you received a copy of the paperwork confirming deposit paid, what are the terms and conditions. Does it mention that any deposit is non refundable?
If however, the purchase was subject to finance and you cannot now get finance, the dealer would have to refund your deposit. But they would be allowed to retain part of the deposit to cover reasonable costs. If you refer to Trading Standards, they will confirm that any purchase that relies on finance, cannot be enforced, if you are unable to obtain the finance. The finance did not need to be with the dealer concerned. The following thread may help guide you, if you use the reason of failed finance to cancel the order. http://www.moneysupermarket.com/community/forums/t/cancelling-a-car-purchase-42360.aspx
Another thread to help you. http://www.moneysupermarket.com/community/forums/t/i-put-a-1632000-deposit-on-a-used-car-can-i-chang-41642.aspx
In regard to Insurance, it may be that the cars group rating is too high for your age. Other Insurers may still be willing to cover. Try a decent local broker (not Swintons). Adrian Flux have various policies available which might suit your requirements.
You would be best to try to find Insurance elsewhere, rather than lose any part of the deposit, if this could be avoided.