The last couple of years, I've stayed at the hotel on the Friday night**, so I've been able to park at the hotel. (And I get to eat a decent breakfast before the fun starts.
)
** - The roads are so bad from Dorset to Bristol, that I'd have to set off at some ungodly hour (missing my breakfast, as eating early in the morning doesn't agree with me) to get there for the start of the Con. (Even on the Friday afternoon, it takes nearly three hours to reach the hotel, partly because of the roads, partly due to the horrendous traffic.) On the way home on Saturday evening, I've returned home via the M4, A34, M3, M27 and A31, which although much farther, takes about 2 hours and there's less chance of ending up in a hedge***.
*** - The A350 is supposed to be a trunk route, but one pair of sharp bends near Iwerne Minster also carries a warning about mud on the road; it's like being on a metalled farm track. (To avoid the A350, I made a detour through Salisbury last year, hoping the route between Southampton and Bristol might be up to scratch. It wasn't.)