Hi Will,
As Sean mentioned, you can go for the flat rate which is around 11% for Window Cleaning I think. You can still claim for some stuff with this if my memory serves me right. I looked into this a while back as I'm approaching the threshold too.
I was looking into setting up a Limited company as well though and being a Director of that company, putting all my expenses through it, therefor releasing the pressure off my current business (but obviously maximising taxable income), and then running the limited company to the VAT limit before merging the two under one name to make the VAT pill a bit easier to swallow.
I'd need to speak with an accountant before taking action on that though as there are a lot of what if's.
Might be worth a thought though?