For charity events I suggest we don’t charge a fee within Swansea area, and we only charge a small fee for traveling costs outside Swansea area (e.g. £25-50).
For events that sell tickets, I would propose a lower flat rate (e.g. £50-100) for smaller events, and a higher flat rate (e.g. £200) for bigger events and festivals, plus a flat rate of travelling costs (e.g. £50) if it is outside Swansea area.
Then we give the travelling fee to the drivers, and the gig fee could be split between the players and Aber Taiko account. This seems a good deal for all to me; whoever turned up to play gets something, but the group gains too from every gig we do.