To get cheaper tickets try splitting the tickets up. For example: Worksop to Cardiff return for 2 adults (1 YP railcard): £132.00 Worksop to Nottingham return; Nott'm to Burton-on-Trent return; Burton to Brum returm; Brum to Gloucester return; Gloucester to Newport return; and Newport to Cardiff return (2 adults with 1 YP railcard) = £76.25 You don't have to change trains just keep showing different tickets. Might be worth trying for people who haven't got their tickets yet.
split the Derby to cardiff bit into derby to brum, brum to gloucester, gloucester to newport and newport to cardiff. Might save about £5 to £10. Go to www.nationalrail.co.uk or www.thetrainline.co.uk
We are going via Sheffield and Derby costing £68.00 quid (£33 for kids) and arrives cardiff about 11.30.
I looked yesterday and found if I booked a single from Cardiff to Northampton, it was £10. But as you need to change at Birmingham for this, you could just get off there and catch another train onto where ever...
RE: Will I be classed as an adult Im 16? Don't know. Check the national rail website or train operators website.