Train Pembroke to London

If it's a weekend then long term engineering is taking place on that route. Fly from Ireland perhaps?

I just do not understand why anybody arriving in Pembroke from Ireland would wish to get on a train and head straight for London. You are arriving in one of the most attractive parts of Britain - Pembrokeshire.http://www.visitpembrokeshire.com
When you arrive on Irish Ferries, you will have to catch a train that will travel along a winding branch line. About 25 minutes out from Pembroke/Pembroke Dock, it will arrive at the very attractive seaside town of Tenby. I would consider staying here for at least 1 night.https://www.visittenby.co.uk
This island is just off Tenby and local boatmen run a shuttle to the island:> http://www.caldeyislandwales.com

A pay on the day one way ticket from Pembroke to Tenby is £5.80. However, be sure to pre-book any longer journey to get lower prices. It is possible to get from Pembroke Dock to London (Paddington) for £26.70 or Tenby to PAD for £26.50. Engineering works are most likely on Sundays. See diagrammatic map:> http://www.nationalrail.co.uk/static/documents/content/routemaps/nationalrailnetworkmap.pdf

Anyway, the train will continue through the countryside of south-west Wales for around 2 hours before it reaches the City of Swansea. It is here that you will see a fast GWR express train waiting to speed you east to London. You must transfer quickly here from one train to the other. The first train is unlikely to offer seat reservations and is not run by GWR but by Arriva Trains Wales - although you can purchase one through ticket for the entire journey. It is probable that GWR will not offer seat reservations because it cannot be done for the first leg of such a journey. Anyway, not to worry as it is most unlikely that either train would not have seats at the places where you board. (I boarded a 10 coach GWR express train in Swansea last Oct at around 3pm and only about 24% of the seats were taken).
You would however, have to check that none of the seats were reserved for later legs of the trains' journey - though at Swansea, it should be pretty easy to find unreserved seats for the entire trip. If it is one of the older High Speed Trains - I think that coach F will be your best bet. However, don’t hang around boarding at Swansea as you can always walk through the train. (If it one of the newer trains, you may find it is divided in two as 5 coach units making 10 coaches total).

I would also consider staying over in Cardiff - possibly in addition to Tenby. Cardiff is about 3 hours on the train from Pembroke and about 2 hours from London.http://www.visitcardiff.com
These places are in the Cardiff area:>https://www.cardiffcastle.com
https://museum.wales/stfagans/
http://cadw.gov.wales/daysout/caerphilly-castle/?lang=en

If you wish to fly, check out www.skyscanner.net (You can fly from Ireland to many regional airports in Britain - including Cardiff & Bristol). Heathrow might be expensive due to landing fees. Look for other airports in the London area.

Would it not have made more sense to buy a Sail-Rail ticket for the entire journey?

Looks like the SailRail tickets are not available for the Pembroke route?

Sailrail is available via Fishguard.

I tend to agree if not taking the advantage of stopping off this is a very long winded way to London, but I guess for some the travel is the thing not the getting there. Stopping for one day and traveling on the next would reduce the stress level for me at any rate.

There is work on this line at the moment somewhere more or less every weekend in connection with electrification or something else and on weekdays too less frequently.

“Do people typically have issues with this line?” I would imagine that the answer is no, as most people arriving in Pembroke wouldn’t be trying to leave one of the most picturesque parts of the U.K. as soon as they have arrived! I would imagine that 99% of people heading Ireland to London would fly, as it’s much quicker.

Engineering work sounds like weekend travel and line maintenance, which is common, but they rail companies will provide alternative means to get you to your destination, but it may take longer.

This is at the tail end of a 3 week tour of England, Scotland, and Ireland. Originally we had thought about flying back from Dublin to London. The Rosslare to Pembroke to London option at least allows us to see the area as we travel through it. Our travel day is June 21, and a GWR person told me that the engineers may be working on the line sometime between the 18th and 22nd.

GWR.com says:

The Patchway and Severn tunnels will be closed for electrification work from Saturday 16 June to Friday 6 July, inclusive.

As a result, long distance services between London Paddington and South Wales will run to an amended timetable, and buses will replace trains between Bristol Parkway and Newport.

between London Paddington and Swansea, services will be diverted adding around 40 minutes to the usual journey time; there will be one train each hour at off-peak times; there are no through trains on Sundays 17 June or 1 July

It is probably because of this that no seat reservations are being issued yet. I guess that the Wales services are being diverted via Gloucester. The number of trains into Wales will be half the usual number except at peak times.

Reservations aren’t yet open 18 June to 29 June on the London to South Wales route

https://www.gwr.com/plan-journey/tickets-railcards-and-season-tickets/booking-tickets-in-advance

Once available you should be able to buy an Advance ticket that includes a seat reservation.

The GWR journey planner states there’s a train from Pembroke Dock at 13:17 which arrives at Paddington 18:30.

If it is the case that the Severn & Patchway tunnels are closed, then the odds are that the train will go up to Gloucester and then take the Golden Valley line via Stroud & rejoin the direct route at Swindon.

When on the first train, please note that it will change direction of travel at Carmarthen. (The best views will be on the right).

At Swansea, the coach nearest to the buffers will be the 'Quiet Coach’ = no talking or mobile phones etc. If, as seems likely, you will have one of the new Intercity Express Trains, the Quiet Coach is Coach A + G. On these trains, you will find large luggage storage in coaches D & K. These trains will have an electronic display to show which seats are available. A green light will show those seats that are free for the entire journey. You must press the button (when lit) to open the exterior doors - which are normally closed.