Thanks all for your response!

• Called and emailed easyjet, they said I should rebook
• Called the LGW airport immigration. The officer was very polite and helpful by the way. He said I can't stay too long in the airport as securities might be asking questions
• Emailed the UK Embassy in Singapore, since they're not processing Visa here and need to send it to Manila, they advised me to rebook

Now I can go on and rebook this flight.

I am the OP. This is what I did:

• Called and emailed easyjet, they said I should rebook
• Called the LGW airport immigration. The officer was very polite and helpful by the way. He said I can't stay too long in the airport as securities might be asking questions
• Emailed the UK Embassy in Singapore, since they're not processing Visa here and need to send it to Manila, they advised me to rebook

Now I can go on and rebook this flight.