If you have ordered as an end user and not a business the following should apply
I'm quite sure you have the right to cancel the order and get a full refund including delivery as long the items are returned to the supplier within 30 days unused and not damaged which obviously they are if you rejected the delivery. Notify them that as the delivery was not complete you rejected it and want a full refund as you need to source elsewhere. If they say the other tyres would arrive next week tell them that it is not acceptable.
Any problems contact Consumer Direct who are the consumer trading standards, they are very helpful.
As a last resort hopefully you would have paid via a credit card which will give you some protection, if they won't refund you in full contact your credit card provider.
Without prejudice these are my own opinions