Testwood Motors still exist. But last time I was there they seemed completely unaware that they seem to have sold about a quarter of the Messerschmitts around. I have probably 5 ex Testwood cars myself, yet I specialise in Kings Motors cars. I do not recall Testwood doing other microcars, but they would have sold trade ins from people uprating to Schmitts, ponce, strut, sneer.
Even Testwood failed to get some special orders. So the Sport and odd colour combos had to be collected, by order, from the factory by Claude Tipper and others. So Testwood were not the be all and end all of Schmitting, despite their dominate position in marketing the cars in the UK.
My fav Schmitt dealer plate is N J Whisker, as no one else seems to have one, or a record of them. Was from a very early car, 1955.