Train from Luton to Cardiff, Wales

Try this site
http://tickets.southernrailway.com/sn/en/journeyplanning/mixingdeck I put in a date of April 16 and got fares ranging from 27gbp to 299 gbp Hope it helps

Thanks! That website worked. It's not as expensive as I first thought from the other website, so that's even better! :-)

April, If I had gone through with it, I could have booked tickets on the East Coast site so I don't know why it wouldn't work for you. However, I'm glad the Southern Railway site did work. At the risk of stating the obvious, here are some things for your consideration: 1. If you book discount fare tickets, they will be for a specific departure date and time and are non-refundable and non-exchangeable. If your flight into Luton is late and you miss your train, you will be out the price of the tickets. 2. The name of the train station is Luton Airport Parkway. You will need to take a shuttle bus from the airport to the station. 3. How you get from St Pancras (where you will arrive from Luton) to Paddington (where you will depart for Cardiff Central) will be up to you. Tube or bus will be your cheapest options. 4. Pay close attention to the total travel time for getting from Luton to London and then from London to Cardiff - and the return. Since you plan to make this a day trip, your time in Cardiff with your friend will be quite limited.

Hey Tim, Thanks for the tips. Nothing is really too obvious to me as this will be our first trip to this part of the world. We will actually be arriving on the day prior and staying with friends in the Luton area, so we won't have to transfer from the airport. That also means that purchasing a non refundable ticket with specific times won't be a problem. Actually I assumed that Luton was the closest, but looking I can actually book a ticket from Harpendon which is where they live (and we'll be staying), so that's even better. I'm not sure what you mean by: "How you get from St Pancras (where you will arrive from Luton) to Paddington (where you will depart for Cardiff Central) will be up to you." Is this a change in trains on the way there? I did see that you had to change trains twice, but I didn't see where or a map, etc. Would they have you make your own way from one station to another to change trains?? I wouldn't expect that. Is there a place to find that info on the website? I do know our time there will be limited. If we take an early train we can be there about 10:45am & leave there about 6pm. It's just the best we can do with scheduling.
Thanks for all the info! I really appreciate any advice. :-)

April
I should leave this to Tim to clarify. London has a number of different railway stations at various corners of the City. St Pancras (where you arrive from Luton) is a different station from Paddington where you leave to go to Cardiff and the west. They are at least a mile or two away from each other. So you will arrive at St Pancras and have to get yourself to Paddington. This would usually be done by Taxi or Tube. Either way it is going to take you some time. Similarly, on your return you will have to get yourself from Paddington to St Pancras so you can catch the train back to Luton. Check the map of London to understand what we are saying- also check the Tube map, you will probably want the Hammersmith and City Tube line between these two stations

Brian has given you the basics. To get from St Pancras to Paddington you can take the Tube (London Underground), a taxi, or the bus. The Tube will be the fastest. The ride itself takes 13 minutes, but you'll have to allow time to get below ground at the King's Cross/St Pancras Tube station and above ground at the Paddington Tube station. Next will be a taxi. The bus will take the longest. Bus #205 goes directly from St Pancras to Paddington. In normal traffic conditions the ride is scheduled to take just under 40 minutes. Go to the Transport for London site to see a Tube map. Click on the "Getting around" link on the home page and then on the "Tube" link below the "Maps" heading.

Ok, I see now. That makes sense. So if you're buying a ticket with a time on it do they allow for time to get from one station to another or is the time on the ticket just your initial departure time from Luton or Harpenden?

You will actually be buying separate tickets for each leg (Harpenden-London and London-Cardiff). It will be up to you to allow enough time to get from St Pancras to Paddington to catch your train there.

Ah, but if you buy a through ticket the rides on the Underground are included. Has to be a through ticket though, and that may not (I haven't looked) be the cheapest possible way. Look at your ticket, once you get one, and the routing in the via box should say London, and there should be a little cross next to it. That little cross gets you one single Underground journey in each direction between the station you arrived in and the station you depart from. At St Pancras you want a westbound Circle Line train or a westbound (same track) Hammersmith and City Line train. You feed the ticket in the reader at the gate and it pops back out above. TAKE IT, you will need to do the same thing to get out, and on the next train to Cardiff. If you are worried ask one of the people wearing a light blue uniform and a peaked cap who will be standing at or near the automatic gates. Once you are on the Underground you will stop at: Euston Square Great Portland Street
Baker Street Edgware Road and Paddington 4 stops, at around 3 minutes each I would estimate around 12 minutes travel time, not counting getting to platform and waiting for the Tube train and getting to your platform at Paddington. The distance is further than some think, over 2 and a half miles.

Nigel, I didn't realize that booking as you described it includes a Tube ticket. I thought the site only suggested taking the Tube between the train stations. Thanks for teaching me something new!