If this wedding is in the centre of Bath and I did not already live in the locality I would do one of these.
Stay at Bath Marina or Newton Mill and use P&R
OR (the wildcard)
Stay at Bristol CC site which is a quiet site smack in the centre of the best of Bristol (tourist wise) and on the day of the wedding I would catch the river ferry to Temple Meads and get a rail return to Bath (Bath station very central).
You would probably need to get a taxi on the way back if the ferries have stopped, but this would be a day to remember for reasons apart from the wedding!