Oh, I see. Thank you
So I should be OK with the Hanwell ticket if I manage to get hold of it somehow?
Also if you stopping the night before, you could change at Banbury, and get the 1837 GWR Service, which runs through to Paddington and calls at Southall on the way. By doing that, you avoid going via London. The ticket combo you mentioned (not via London) would be valid for this route and since you plan to arrive at Heathrow in the evening, the timing of this service should fit in with your plans to stay at Heathrow overnight.
So you could do
1G45 1330 Aberystwyth - Birmingham New Street arr 1630
1O24 1704 Birmingham New Street - Banbury arr 1754 or 1H68 1710 Birmingham Moor Street - Banbury arr 1800
2P82 1837 Banbury - Southall arr 2043
2T72 2049 Southall - Heathrow T4 arr 2110
Otherwise, you need a via London ticket if you want to travel via Marylebone, but its something to bear in mind and you can still avoid XC