Update: You can now order a Model 3 in the UK directly- just follow this link: https://ts.la/ben60441
If you ordered on the 1st of May, you can still follow the steps below to have it retroactively applied!
Currently you can’t order a Model 3 in the UK and benefit from the 5,000 free supercharging miles.
I spoke to Tesla UK and was told you can email firstname.lastname@example.org (tapping this link will prefill the email for you)
After ordering send your RN (reservation number) to that email and referral code BEN60441 to get 5,000 miles free supercharging.