A. A1 is a natural colour but it does has a yellow undertone to it. It isn’t the whitest in the natural shade guide and there are porcelains that come in much lighter colours. Your dentist has probably made them in A1.
Veneers can be placed on top of existing ones and also ontop of crowns but it doesn’t normally end in a good aesthetic result. The teeth will look thicker than they should and will feel bulky.

It is your choice however, as long as you are realistic about the end results.
If you are really not happy with the colour and would pay for veneers to be placed ontop of the existing ones, I would have the old ones removed and new ones paced. Have it done properly and don’t cut corners because you may not be happy with the end result.

I am guessing that you went to a NHS dentist to have them done based on the price. when I have veneers made for my patients the lab bill alone is twice as much as the price you paid for your veneers. You have had an extremely good deal but I must also say that you get what you pay for and if you want a pleasing aesthetic result you will need to be prepared to pay more and go to a dentist that specialises in these procedures
Kind Regards