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.