I've had this problem with the east coast.
I've paid and launched from the slip in whitby and parked up near the slip, and like you said its expensive.
I've seen the slip at Sandsend and there's a car park next to it, but you'd need to be early to get a space.
About 10 years ago I went to Filey and managed to park on the front at the far end away from the official slip, inflated my boat on the grass and kitted up to walk down one of the other slips across the beach to launch, when I was stopped by a woman with a walkie talk and told I would need to go pay my launch fee and launch at the official slip. I walked the length of Filey beach with my boat to meet an old man in a wood shed who gave me a list as long as my arm and said "if you haven't got any of these you aren't launching". I didn't have a fire extinguisher, bucket, compass, so I couldn't launch. He told me if I went to Scarborough south bay I could launch there. What's the point of that!
I went to Scarborough kitted up and went all the way to Filey in my zodiac cadet 240 just to make a point. Once there I bombed about the bay waving to the old t...t looking with his binoculars.
If you find any good free slips round Yorkshire, please post!
PS. Cornwall is great for launching for free.