The rule to long term investment success is diversification. You should always have an amount of cash available to you in case you need it in a hurry, but no single asset class should be relied upon once your portfolio starts to build beyond those emergency requirements. Equities, bonds and property don’t offer the same guarantees, but if you understand them and are happy to accept at least some volatility, the longer term value of your portfolio might be better for spreading at least some of that money around. 

NB: If an institution with which you hold deposit savings is declared insolvent, the capital will be guaranteed by the Financial Services Compensation Scheme up to a maximum of £85,000 per person, per institution. If you hold more than £85,000 on deposit, therefore, it may be sensible to spread it across more than one institution.