Colin - if you are actually traveling to the US at some point its pretty easy to get a blood test for siriolimus - you can easily order online with just your name and a credit card, and then go to the closes Lapcorp or Quest facility for the blood draw.
If you’re trying to get the blood test done by a US company when you are in the UK or other country, then yes, I suspect thats very difficult because the lab company has to do the blood draw. I don’t know if any of the companies like Quest or Labcorp have blood analysis facilities outside the US, the rules and regulations are so different in this area that I suspect not: How to get a Rapamycin (sirolimus) Blood Level Test
I tried it last time I was in New York and had the labs lined up but the blood draw company would not allow a non US to have their blood taken. If I cannot find UK company that does this test then I understand that I can get a UK private doctor that is registered with Quest labs to take the sample and get the test done.
That seems really strange… I can’t understand why they would care. Perhaps try a different lab (contact them before you go) and determine if this is something that is limited to that company, or that lab, or that city/state, etc. The US is one of the top destinations for medical tourism so I can’t believe that this is an issue everywhere in the US.
New York State doesn’t allow it - I think there are two other US states that don’t allow it though I don’t remember which ones. It’s possible in all the other states.