Any cash that you deposit into your ISA this tax year counts towards your annual ISA allowance, even if it hasn't been invested yet. This is true for payments that you make towards your ISA portfolios or ISA Account cash with InvestEngine, as well as for any payments you make this tax year with your other ISA providers.
ISA transfers can include both this tax year’s payments, as well as the balance of your previous years' ISAs, as per your choice on the transfer form. However, only payments made in the current tax year count towards your ISA allowance.
The cash balance will remain uninvested only if you are using a DIY Portfolio with the AutoInvest feature turned off.
This feature may be useful if you want to reduce your exposure to the market and employ the PCA (pound-cost averaging) strategy, where you invest only a portion of your total planned investment budget every month.