It is very possible to fail the Isihara plate test, and then pass the Lantern Test at CAA Gatwick. Have you exhausted this route?
If you pass the LT then show the letter from the CAA AME to the FAA AME and he will give you a Class 1, 2 or 3 medical. PM me if you want any FAA AME contacts.
I wouldn't bother with FAA Class 3; the Class 2 has the ECG and costs an extra 20 quid and avoids the mounting controversy over the Class 3.
The LT is pretty unrealistic for real flying needs but you can practice for it, with coloured lights about 5mm dia about 5m away. Also avoid doing any close viewing of things immediately before e.g. reading.