The garage wont guarantee the part. The company who makes the part will guarantee it as far as I know. I could be completely wrong so someone correct me if so, if the part fails due to incorrect fitting it will be down to the garage, but if its down to a part failure within the guarantee time period it will be down to the part manufacturer to replace it.
See this from the Clarkmotorsport Terms and Conditions
"All guarantees for new products are provided entirely by the manufacturers/UK importers and are subject to their terms and conditions. If you wish to pursue a warranty claim we shall act on your behalf and shall treat all claims as a matter of urgency."
For example, if you buy an exedy clutch and it fails, if you had bought it from clarke it will be down to exedy to replace it however clarke will pursue the claim for you.
All this said, go to a decent garage and you really shouldn't have any issues, you seem fixated on dealers and warranties but what I would be spending more time researching is which garages are best for the job, this has already been answered for you a few times, Horsham developments for example. Any trader on here will not want to ruin there reputation within the forum by selling or fitting the part wrong on your car.