I'm not local to you, but my experience in southeast England is that it is rare I can get to the the start by public transport.
300km events? start at 6, so overnight required if not driving.
Sunday events? early trains don't get there in time.
Same for me here in East Kent. Weekend trains used to be much better but the service has been badly downgraded over the years.
It doesn't help that there's never been much of an Audax scene in East Kent. If you really want a good selection of events without having to travel great distances, you need to live in an area where there is a thriving local Audax-focused club - ACME in Essex, for example. Wrong side of the estuary for me, unfortunately.
When I first started, there was the Kent Corners 200 from Canterbury, so I could easily ride the 8km to the start, but then the organiser moved out of the area. Aside from that, the only events in East Kent are the "Fairies five" from Bethersden. If I wanted to do the 200, the earliest train from Canterbury on a Sunday wouldn't get me there in time, which is ridiculous considering it's only 20 miles down the road. For the 300, the 2am start would mean getting the last train on Saturday then hanging around at the village hall for a couple of hours.
The dearth of rides in East Kent is one of the reasons I started organising one of my own, but I've not been able to put that on for the last few years. I tried to get my club involved in that but most of them are only interested in time trials.
There's a better choice of rides in West Kent - Tom Jackson's Hop Garden 200 and Oasts & Coasts 300 from Meopham, but getting the train to the start for those is a similar story to the Fairies events. David Winslade's Man of Kent 200 from near Tonbridge is even harder to get to by public transport.
For all of the above rides, I tend to ride to the start if I feel up to it. In the case of the Man of Kent, that's 65km. Which isn't too big a deal for a seasoned audaxer, but aside from adding significant distance to the event, it also means riding solo through the night to get there - it's a nice ride but I can see it's not an option that will appeal to everyone.
I'd like to do more events further afield, but when you start having to factor in cost of accommodation the night before, it becomes a much bigger undertaking, so I only do it for events that I think really justify the outlay - which usually means nothing shorter than a 400.
Not that I've done any events of any distance for ages, so it's a moot point. I really ought to pull my finger out and get my event back on again.