The Severn Tunnel will be closed for engineering work for a good bit of July. This tunnel under the estuary links Bristol with south Wales. It will add around 50 minutes to journeys between London and south Wales with trains coming off the direct route at Swindon and heading on the south Cotswold Line to Gloucester and then down into south Wales and vice versa.
The bigger problem will be for those going between Bristol/Bath and south Wales (Newport, Cardiff and beyond).
Details:>https://www.gwr.com/travel-information/travel-updates/planned-engineering/bristol