Can I board if my flight arrives 1.5 hours before my UK visa becomes valid?

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.

The sterile corridors at Gatwick before immigration are open even at night, as there are a number of nighttime arrivals, so in theory you could go to the toilet and wait until midnight before heading down to immigration (although you'll likely be noticed by the cameras and have to explain why you're doing this - in this case be honest)

In practice, it is unlikely that the airport staff in Venice will let you board your flight, because the UK could heavily fine easyJet for transporting you too early. With easyJet being a budget carrier, they will be particularly careful about this.

Furthermore, your passport info gets sent to the UK in advance, so the airport may get a "DO NOT BOARD" alert from them.

If I were you, I would re-book the flight immediately, or apply for a change of dates of the visa.

my flight to LGW is June 24 10:30PM

If that is your departure time from Venice you have 15 minutes to spend walking to immigration after landing. That is in perfect world, not calculating any possible delays during disembark. The flight is 2h15m, if you factor in timezone change, you should land at 2345...

I would still check with EasyJet whether they will allow you to board though. My uneducated guess is they will not.