Did the organisers of the festival attempt to arrange sufficient extra transport with First Great Western? Or did they simply organise a festival and expect everyone else to sort that problem?
I have a lot of experience in coordinating local services to facilitate large events such as music festivals - the planning and delivery are mangaed through Safey Advisory Groups (SAGs) which include the local authority's licensing, planning, highways, environmental health, trading, child protection, public health and estates departments, all of the emergency services, and several others. It takes months of planning and regular meetings. And there are always debriefing meetings. These are always chaired by the local authority, (and I've had plenty experience of event planning in Bristol which I can confirm conforms to this procedure). None of what has been said on here will have escaped the attention of the SAG members.
If there is anything that you really believe might not have already been brought to their attention, then please do contact the Events Licensing Officer at BCC, who will be chairing the SAG.
There's nothing to be gained by having a go at the rail operator. As a personal note, I believe I must be exceptionally lucky, because on my regular travels I suffer very few delays, but I know that incidents are frequent and do lead to delays of more that one hour quite often. That's UK transport. Trains can become crowded to absurdly dense degrees. That's UK transport.