Yes that's perfectly allowed, although its worth noting that the 0800 ex Cardiff isnt a 143, its normally a 3-car 158.
Not sure about changing where the booking engine makes you change trains, you could try forcing it 'via Exeter St Davids', but i'm not sure otherwise.
Must have got my classes muddled up - although from my notes the last couple of times I have caught the 08:00 it has been a 150 (150106 and 150121)
Sites powered by WebTIS (e.g. Southern or Red Spotted Hanky) automatically suggest changing at Exeter St Davids for this service.
I did find a way of forcing WebTIS to issue changes at a different point, but I cannot seem to get this to work again. It used to be very handy for my Milton Keynes to Portsmouth journeys as I could avoid changing at the horrid/noisy/unwelcoming Clapham Junction and instead change at slightly less unpleasant East Croydon.
Thanks I will give that a try - many thanks and congrats on your promotion.