What are you planning to do up there that those posts are stopping you doing? You're in danger of creating a habitable space, some Councils would argue you already have, if you go past that tipping point you're required to comply with Building Regs, which that space clearly doesn't. Realistically, it isn't going to cause you a problem, but it could if you sell the house in future as surveyors and mortgage companies are becoming a bit more switched on when it comes to dodgy loft conversions. No lender, no buyer. There's also a risk that the house wouldn't be insured in certain circumstances.
Given whoever did it has left the posts in and not done anything in respect of safe access, there's a pretty good chance they didn't bother to construct a safe floor or insulate it properly, which means removing the posts isn't going to get you a great deal. From your pics, if the roof is that steep, I would guess you haven't room to bring in acceptable stairs from anywhere. You would need a dormer to get the necessary headroom.
I would leave it as it is, or rip it out and start again, but probably leave it as it is.
Your question is a bit of a how-long is a piece of string question. I've removed a King Truss from our house, I added a new ridge beam using a glulam and reinforced all of the roof rafters, but that means ripping everything out apart from the floor. Cost me maybe £500 in materials and a load of my weekends, but you need a structural engineer, so add a grand for that and then you need to replace all the mess too.
