If you invest into a taxable investment such as a collective, you will pay tax on gains at 10% or 20% and tax on income at 7.5% or 32.5%. There are annual allowances of £12,300 (on gains) and £2000 (on income).
Cash ISAs are a total waste of time. A basic-rate tax-payer can earn £1000 a year interest before they pay tax. At a savings rate of 0.1%, you would not pay any tax unless you had more than £1,000,000 on deposit. A higher-rate tax payer could hold £500,000 and still pay no tax. So what is the point of a Cash ISA?
So if you want to boost your long-term savings by removing tax, invest into a Stocks and Shares ISA.
You need to invest by the end of March as most of early April is the Easter holiday!