There's really no need for train crew changes mid-route on a line that is only 3 hours long – if they were done at Paddington and Swansea instead, no-one would even notice them. Sounds very much like the crew booking on place is in the wrong location.
That is fine as a principle but you need to ensure all Swansea crews end their shift in Swansea and Paddington crews end in London. If you can't then splitting a journey between two crews may be a way of achieving it.
Two-day rosters with a hotel room in the middle would come before that in aviation. Thankfully the days of quasi-random for-your-inconvenience mid-route stops at Shannon Airport types of location are long in the past.