Can anyone tell me where on the in game map is Swansea as I can't see it on load up map or in game map as it's one of the service mode runs
Swansea is not included. It features Reading to Paddington and thats it. In the Menu it just describes where the Service would go IRL, but you can only drive the included modelled section.
It sucks but i expect it will be extended in time. If you want a service where you get to stop the hst at slough use one which is going to Worcester or Hereford
If train simulator has shown us anything it won't be extended. Chances are there will be future dlc that goes further than Reading though . They dont generally extend an existing route. They always release an extension on a seperately route so you won't be able to do the full journey in one go unfortunately
It won't be extended, partly because of their schedule (focusing on different routes) and partly because of technical reasons.