I'm in agreement with the others that it would make sense to discuss this with a professional who'd be able to look at your situation holistically as there isn't really a one-size-fits-all answer.
Like William, I'm a financial adviser and if I was talking to a client, I would be looking at aspects such as:
- how long you want to invest for and whether you'd potentially need to access it in the short term
- your goals and concerns
- the sum in relation to your other assets
- how best to use tax allowances available to you
- how you feel about risk
Without knowing all of these things, it's very hard to say what's right for you. Here is some generic information which may help:
Cash ISA
You have £20,000 ISA allowance each year. A cash ISA has tax advantages: no tax on the interest and no tax when you take it.
But because interest rates are low at the moment, and particularly since the government introduced the Personal Savings Allowance* you may not need to pay tax on your cash savings anyway. (*See
https://www.gov.uk/apply-tax-free-interest-on-savings).
That said, there are other reasons that a cash ISA may be appropriate - not least capturing the 2018/19 ISA allowance which you lose if not used by the end of the tax year.
"Fixed bond"
A savings bond gives a fixed rate of interest over a term - and typically the longer the term, the higher the interest rate offered. But check the T&Cs to see whether:
a) you actually can't access it sooner than the end of the term you choose
b) you'd forfeit (some of) the interest if you needed to access it
RateSetter
This is a different proposition as it's not cash savings, it's peer-to-peer lending. You get potentially higher returns because the risk is higher. RateSetter is regulated and authorised by the FCA (very important) but they are not covered by the Financial Services Compensation Scheme.
I hope this helps.
