if you decide to up your prices by 20% over night
You don't "up your prices by 20%" because that ignores the VAT you can now claim back on your purchases, such as fuel, equipment etc etc. After that is taken into account, its more like a 15% increase
A 15% increase on a property charged at £20 is £3.
I'd pass that cost on and not even worry about it. Or possibly absorb half and pass on half for a year, then pass it all on after that.
Also, Crystal, you are over-thinking this. If you're nearing the £85k turnover then you're also in a turnover bracket where proper tax planning is worthwhile, and you can save more than 15% by going Ltd and implementing the many tax-reducing options that being a corporation opens up.
Personally I'd never ever let a fear of losing 15% to the tax man limit my ambitions or push me into a business model I wasn't suited to (such as going for commercial) nor would it prevent me from continuing to grow.
I also would not try to come up with some dodgy scheme to avoid it (like setting up two companies LOL they are all over that old trick). It's just a step on the path of growth that you have to take.