£650k is a pretty chunky amount of money, and enough to secure a comfortable life going forward, if you get things right.
I don't know your circumstances: age, marital status, children, debts, mortgage, pension arrangements, etc. all of which are crucial in devising a suitable plan. So probably some professional help would be useful.
But...
What sort of help ? There are broadly two kinds, which people muddle up:
Accountant versus Financial Advisor
I suggest that an Accountant's advice (paid by the hour) is invaluable, as they will be able to steer you into tax-efficient and economically-sensible strategies, such as paying off debts (and maybe mortgage), maxing out and organising pensions (with beneficial tax breaks), investing tax-effectively (ISAs), providing for kids (Junior ISAs, pensions), explaining why trusts are expensive and complex overkill.
Financial Advisors are a different and dubious breed. I don't have the word count here to go into everything that's wrong with FAs, so I suggest you look up "St. James Place" to read about all the shenanigans that FAs get up to; the root problem is that their interests are certainly not your interests.
After 40 years of personal investing I can say this:
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a twenty-something commission-hungry flat-sharer wearing an M&S suit cannot beat the market, no matter what they tell you. When FAs try to sell to me, I like to torment them by asking for a photograph of their presumably big, luxurious house, as would befit a master investor
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it's almost impossible even for professional Wall Street types to beat the markets, so the best approach for all normal small investors is to buy a low cost global index tracking ETF, and let it run unmolested. Consider, for instance, Vanguard All-World UCITS ETF (VWRL). Look up "John Bogle" for extensive justification and history of godfather of this philosophy.
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Consider the implications of the often-confused concepts of "risk" and "volatility", and how they interact with time. Equities are surely volatile in the short term (say up to 5 years), but equity returns smooth out over the long term (10 years+). Conversely the "risk" of your investments dwindling due to inflation if held in cash or bonds approaches 100% if held for the long term.
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In investing there are two factors that you can control: tax and fees, and two that you can only do your best to manage: risk and returns.