Swansea (Welsh: Abertawe) is a coastal city and county in Wales.
The M4 motorway crosses though Swansea (junctions 44 to 47 inclusive).
Swansea service station is a complete hell hole to get out of if you are going east (say towards London). Some anonymous hitchhiker always had to resort to asking truck drivers who are waiting in the services on their tacho-breaks.
You will want to get onto the A483 to leave Swansea and head East - If you walk from the city centre out to Fabian Way (Over the River Tawe and between St Thomas and the new shiny Marina Development)
Northwest towards Carmarthen
From the Swansea train station walk to Camarthen road (the journey takes 10–15 minutes). Hitch from there to the service station on the A84/A483 and M4 roundabout. From the station you can find lifts to Camarthen and further to the Pembrokeshire coast