It asks for the VAT rate in the owners equity page, so how do I record the "tax' rate for taking drawings from the business, based on a tax code of £11,850?

No VAT, taking your own money does not affect VAT.

Not sure of the relevance of the tax code in your comment, sole traders are taxed on their profits not their drawings. If you're a "one-man" limited company then drawings don't exist, in that case you should be looking to take a salary and dividends.