When registration is compulsory
You generally need to register when taxable turnover goes over the current £90,000 threshold or you expect to go over it under the forward-looking rule. Special rules can apply.
When voluntary registration becomes a business decision
If most customers can reclaim VAT, adding VAT may create less pricing pain and you may recover VAT on eligible costs. If your customers are consumers or non-VAT-registered organisations, adding VAT can make your effective price more sensitive unless you absorb some of it.
What MoneyCue should eventually calculate
Enter customer mix, current prices, VAT-able costs and turnover, then compare staying outside VAT with registering and keeping price or margin constant.