As Ian said, the reserve list on the day is created on a first-come-first-served basis - and last year a friend of mine got to the casino in Cardiff for 9:30am, and was second on the reserve list!
He got in, but this season it"s even harder to get in as the law in the UK now allows alternates (and this means fewer people dropping out on the day as players don"t have to be in their seats at the start of the tournament any more).
If you didn"t want to travel to Cardiff now you know you"re not guaranteed a place, it should be possible to cancel most hotel bookings before the day of your stay without any charge (or maybe a very small admin fee).
Best of luck with whatever you decide to do.