On the outward leg you'll possibly be affected by FCC▸ 's evening peak restrictions.
That shouldn't be the case. The restrictions for the outward portion of a Cardiff Central to Luton airport Parkway are:
Available on all trains departing Cardiff Central at or later than 0940 for the Super Off Peak, and
Available on all trains departing Cardiff Central at or later than 0810 for the Off Peak.
There are no further outward restrictions from London to Luton Airport Parkway on either First Capital Connect or East Midlands Trains.
On the return journey on Tues 26th November you can take any train from Luton Airport Parkway to London, but there are restrictions on the services you can use from Paddington to Cardiff Central. You can only use:
Services timed to depart London Paddington between 1010 and 1501, and from 1901 on the Super Off Peak, and
Services timed to depart London Paddington between 0810 and 1640, and from 1831 on the Off Peak.