You’ll likely only buy ex VAT if it’s an internal transaction within the LEA.
You’ll claim VAT back for goods from other suppliers. Generally, schools don’t sell goods or services so don’t pay output VAT, but you would need to add VAT to the sale of any ‘other income type’ item sold from the school to you; including things like selling yourself the old photocopier.
Generally, a Madison trade account is more expensive than stuff from CRC apart from obscure stuff that CRC don’t discount.
EDIT. Sorry, just seen your post further down where you said you’re not selling them on. Still probably cheaper through CRC