I would strongly advise you get mortgage approval first before you start to build if you are going to need a mortgage, this will ensure you have the proper regs/planning docs/architect or engineer sign off etc in place. You only need to draw down the money when you need it although it depends what you are calling a slow build, a lender will have a time span on a mortgage approval but of course you could always just draw down a small bit once you have some done just to secure the mortgage drawdown.
The worst thing to do is start and be half way through and then try and get funding, what happens if all the relevant bits the lender wants sign off on are not done or done the way they want. If you want bank funding then you have to comply with their requirements so better to know this ahead of time.
All that said if you are going to be short a small amount then there is always the possiblilty of CU loan or similar, less paperwork etc but higher rate.
Also you need proper self build insurance, the stuff that covers workmen etc as if you are doing direct labour you are the contractor basically and the buck stops with you. Most of those policies are for around 18 months so make sure it can be extended if needs be, it will cost you at least 1k and probably more but my point is don't believe anyone that tells you they got it from their bank or some random place for a few hundred. That is not 'self build' insurance, that is basic house insurance that covers far less and is mainly put in place to satisfy a lender and is fine if you are building with a main contractor who has the necessary workplace insurances.