A lot of trains are getting bought now and a lot were in the early 2000s, its just the orders in the early 2000s were going to Adtranz/Bombardier and Siemens because of the bad reliability of the Coradias and Junipers. If the trains carry on to get built well (but they need better software...) then I think Derby will be fine.Surely a UK plant and UK expertise are far more useful, seeing that the UK railway has many unique aspects which make local knowledge useful. It's also a very big market, the UK has bought a lot of trains in the last 20 years.
The draw to a UK plant is that operators like to say that the train is built (even if it is only assembled) in Britain, this means that the train being built/assembled here can influence if the train maker gets the order. However this isn't necessary as Stadler have been getting orders lately however this is probably because the level access makes the trains more compelling especially for the metro operations a lot of Stadlers will be on, the decision to order Stadler trains has still been criticised a little though as, for example, when the Tyne & Wear new trains were ordered there was some complaining that it didn't go to Hitachi which have an assembly plant in the UK.
Talgo plan to export to other countries as well from their Scotland factory (notice factory as it is planned that it will make its own body shells after a few years of just being an assembly plant), I think they said that they just need a major order for the factory and it will start construction, also that this order doesn't even have to be from the UK but could be for the export market.while Talgo are investigating UK assembly too