You can't buy anything via
www.nationalrail.co.uk, it refers you to the train company and no it doesn't work with Arriva or thetrainline.com
www.bahn.ie will do cross europe without much trouble but again it won't book tickets
Guy I work with just 'to see England' went the long way to London via Holyhead last year, booking 48 hours ahead Aer Lingus would have got him to Heathrow for the same money 12 hours faster, travel costs tube/bus more or less the same on both options