1) Can you take an earlier retirement for an actuarial reduction? NHS pension is also DB (defined benefits) like the Civil Service one and they offer this.
2) As you are a higher rate tax payer then SIPP
probably makes most sense for you. The investment vehicle e.g. Dodl will top up what you invest by 20% and then you can claim a further 20% back from your end of year tax calculation. I use Dodl and the funds are invested in the HSBC FTSE All-world index fund - its a bundle of shares in different companies that will largely reflect the stock market as a whole.
3) If you're likely to want the money earlier e.g. in a timeframe above 5 years but before retirement then S+S ISA is the thing to go for and invest in a low cost index tracker e.g. FWRG or ACWI which generally return about 8-11% average annualised return (beating any cash ISA). Disclaimer: past performance not indicative of future performance yadda yadda
4) If you're likely to need the money within 5 years stick with your cash ISA.
Not financial advice etc.
Always read the flowchart.