Have you tried speaking to a manger about this?
Advise them that Lexus Hatfield are more than happy to fit them for you, so why can't they?
Explain that you understand they can't guarentee the parts which is fine by you and make them aware
you are a loyal customer.
You will be very impressed with the brakes.
You may need to change your front springs as the car will dip a lot more when braking.
Note, before you brake make sure the person behind you has enough space stop aswell!
I have had a few people nearly go into the back off me because they can't stop that quick. :)
You luck with getting your brakes fitted
Regards
Paul.