Airfix used to produce some plastic kits of locos & other railway items some years ago.
There was also a company called Kitmaster who produced loco kits including Royal Scot, L&YR Pug & a Garrett(LMS)
I don't think Airfix produce railway items any more & Kitmaster went out of production many years ago.
But look around you can some times acquire some of these kits at car boot sales model shops etc
http://www.kitmaster.org.uk/
http://www.tri-ang.co.uk/Airfix/
If you want an A3 Kit try here (a tad expensive)
http://www.djhmodelloco.co.uk/oo-gauge-list.asp