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.

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.

Lingering in the passageways (or disappearing into the toilets) at Gatwick for 1.5 hours is a naive idea, it’s called “suspicious behaviour” in the contexts of both security and attempting to evade immigration control. For them, it means someone deplaned but did not reach one of the other control points and they will check where the leakage occurred with their cameras.

Since it’s a suspicious event, they will dispatch a unit to make eyes on contact. The unit may or may not include one of the medical cyclists on call around the clock, but for sure would include at least one security staff. They will make a welfare check and wait outside the toilet for you. Unless you are seriously ill they will conduct an id check and escort you to the primary control point for a more intensive line of enquiry.

When it all becomes clear that this was a simple matter of passenger convenience and not a seriously ill passenger, the IO can decide what to do.

Note: the controlling technical reference for this question is Paragraph 31A of the rules, which says…

Where a person has arrived in the United Kingdom with leave to enter
or remain in the United Kingdom which is in force but was given to him
before his arrival, he may apply, on arrival at the port of entry in
the United Kingdom, for variation of that leave. An Immigration
Officer acting on behalf of the Secretary of State may vary the leave
at the port of entry but is not obliged to consider an application for
variation made at the port of entry. If an Immigration Officer acting
on behalf of the Secretary of State has declined to consider an
application for variation of leave at a port of entry but the leave
has not been cancelled under paragraph 2A(8) of Schedule 2 to the
Immigration Act 1971, the person seeking variation should apply to the
Home Office under paragraph 32.

(And note that paragraph 32 is no longer viable)

For your more specific questions…

Will the EasyJet airlines going to allow me on board? I will be coming
from Venice.

This is dependent on nationality and staff diligence and intangeable factors at (I assume) Marco Polo. The answer is indeterminate.

How long does it take from landing to Immigration in London Gatwick?

At 22:30 it should take about 10 minutes max; the queue at the control point may be lengthy but no longer than about 20 minutes for a non-EU arrival.

Can I stay longer in the airport before I pass through Immigration?

See above

Can I still see my luggage in the conveyor belt after 1.5 hours? How
much time before they take away my luggage from the belt?

Once the counts at each point have matched they will pull it and store it, but this will take a much longer time for them to get around to it.

TL;DR

There is no such thing as hiding in Gatwick for any noticeable length of time. One way or another you will need to explain yourself and your options are (1) under suspicious behaviour or (2) passenger convenience.

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.