The UK cannot just "be Singapore". What happened in Singapore was a specific, unrepeatable combination of its geography, the needs of the region, the size of the country, and the culture.
To maintain its wealth today, Singapore relies on a large underclass of underpaid non-citizens. Around 40% of the country are non-citizens.
In addition, London sort of has its own Singapore(s) in the form of the City and Canary Wharf. That's great for those who work there, but it's not feasible for a country of nearly 70 million for everyone to just work in finance.
Final comment:
> Singapore did not depend on neighboring countries to climb out of 3rd world poverty
Singapore's wealth is built on trade and foreign investment. To assume that without other countries it would be equally successful is absurd.