From Heathrow to Gatwick Airport, the fastest is 1.5hr by public transport, or you can call a taxi for about 50mins.
In other words, the aircraft is not delayed, everything goes well, you do not check baggage, and 2hrs can get off the plane to Gatwick airport,
Then the ground is pleased to let you check in 1hr before boarding.
In fact, the general mode of public transport passengers should stay 5hrs above.
That cannot get on how to do? Three possibilities, the first is to transit when the border officials directly to your repatriation, do not let you inbound. Because you show a plane ticket that is almost impossible to get on unless you have enough evidence that you just want to turn around.
The second solution is to change the votes as soon as possible.
The third way is to contact the ticket seller, if you two flights are sold by the same travel agency, on behalf of their homework problems, or airlines approved such a trip, then by that time if you cannot keep up the next class Aircraft, airlines are willing to help you change the votes. But not like this, please ask the travel agency (the person who sells you).
If the airlines (travel agencies) approve of such transit options, they can also contact them to determine the itinerary can be so go.
From gatwick to heathrow transfer to the Italian ticket will be better, because an airport in the west one in the south, take public transport basically should be too late, take a taxi maybe you can buy a ticket
I tried to contact the person selling tickets, this is a special ticket, not for any changes ^^, I consider too much to buy a ticket from LHR to VCE, but the return ticket is invalid!
So you can only rush to LGW in time to go all the best, or buy a ticket from Britain to Italy and return to Hong Kong, the latter cost you can buy a new Italian round-trip ticket! That can only choose the former. Now to consider how to get to the plane and less than the aircraft, the moment how to deal with.
You said BACKPACKER and take a taxi will consider ^^ but not sure enough to catch the plane, I think about it could not catch up on the machine should do it!
From Heathrow departure to the Italian ticket will be better, because an airport in the west one in the south, take public transport basically should be too late, take a taxi maybe you can buy a ticket