Own insurance counts for nothing when working as a labour only subcontractor. Own saw? I'd rather they just used mine, at least I know they'll start. Seriously, it's a bone of contention that people think they're worth double an employed person just for a grands worth of expenses. That's all it costs for a saw and basic kit.
OP is in an excellent thing called the free market. I myself started off in a similar way to him, though on agricultural rather than arborcultural work. With a borrowed MS230, happy days!
OP, fill your week with work, for as many different people as you can. Once you have a solid week booked, up your rates by £10/day. Some people will stop using you. Doesn't matter. If you are good word will get around (especially if you do as I said and work for as many as possible) and you will fill the week with work at £70, all the while whilst building a toolkit to make you worth more. The more people you work for, the more different skills you will accumulate. I remember thinking 'it's got to be a onner a day with a chainsaw' way back when. Looking back I knew nothing. I'll probably do the same in another five years time
Repeat this process until you are earning £120-150 a day as a subbie, and then you will become too expensive for most employers. Either stop there or start taking on your own jobs.
I'm happy to pay certain subbies £120-150 a day, and do on complicated fencing jobs. That doesn't mean that every subby with a chainsaw is worth anything like that however. It's called a free market, find your place in it
I've been spending every penny I earn on tools and toys since age 17. I still remember spending my first pay packet from the farm in the Screwfix catalogue. I still have tools and consumables bought that day and they still earn me money.