The Heljan O Gauge (2-rail) are sold in Britain through Howes Models in Kidlington, near Oxford.
I visited them last year when I was at Oxford University for a month. I remember posting some photos in Friday Photos, around late June 2016.
Their website is https://www.howesmodels.co.uk/
The link to their Heljan page is
https://www.howesmodels.co.uk/subcat/Heljan%20UK
While not on their website, they often have shells with small blemishes, broken parts, etc. I would send them an email .
Most of the engines run about $700 US + shipping + duty.
Lad