In the UK at least the legal contract is with the supplier and the purchaser. If you buy any product it is actually up to the retailer to deal with any repairs. Now in the photo industry at least an authorised retailer is going to use Canon to do any warranty repairs. Along with this, as a service to both the retailer and the customer, many companies will actually bypass the retailer and do warranty work direct. This is though at an added cost to the company that has imported the product and distributed it to the authorised dealer network. This is an extra service that is being offered to you. If you buy a product and that product was not brought into the market by the official distributor, is it really right to expect them to pay for service on a product that they did not import?
Of course if you buy a product from a company based here in the UK that is fine as you still have your warranty, as it is a statutory right, and it will be up to your retailer to put right. If you have a problem with a product sold in the UK and go to your retailer, then they may actually use any repairer that they want to to carry out the repairs. It will usually though be the wholesale supplier of the product that carries out the work, as they will not normally charge the retailer to do this.
One word of warning though, especially when dealing with retailers on Ebay and the like, many of them will seem to claim to be UK based, but when you look at the terms they are sending the product direct from China/Far East. Then you may have a problem as you will have no support from the UK wholesale chain or from your suddenly off shore retailer.