The Doctors Laboratory (the biggest private laboratory in the UK) do TSH Receptor Antibodies (TSI is one type of these). You have to have a doctor’s request you cannot order yourself.

http://www.tdlpathology.com/test-information/a-z-test-list/t

Medicheks (who I believe use The Doctors Laboratory as their lab) also offer this service, but the markup is huge on the price (199). The advantage is that you don’t need a doctor’s request and the result if sent to you.

http://www.medichecks.com/index.cfm?s=2&d=21&test=TSI&name=TSH%20Receptor%20Stimulating%20Antibodies

About TSH Receptor Antibodies.
http://www.palms.com.au/Education/Endocrin/TSH/TSHtalk.shtml