Heathrow to Gatwick. how much is a cab

Pre-booked private hire vehicle: £55 or so for a basic service, more for frills.

Taxi from the rank: Over £100.

Coach costs £15.

Time: allow at least 1.5 hours; could be more or less. You haven't said on what day you are travelling - Monday mornings are the busiest on average.

In addition to not saying which day, you also haven't said which direction you are flying. Good conventional wisdom is to allow at least 2 hours to check in for a domestic (read within Europe) flight, and a good long three hours for a flight to North America. Flights elsewhere in the word vary but I would never want to arrive with less than 3 hours under my belt.

It is not clear from your question whether that 8:30 is from the time you walk through the frosted doors from immigration and customs, or from when your plane is scheduled to land.

I'll assume that 8:30 is when your plane is scheduled to land, that it is an overnight flight and arriving from North America, and that your onward flight is an Easyjet from Gatwick to somewhere else in Europe.

I think it will be very tight, you may miss your onward flight, and it will be very stressful no matter if you use the coach or a taxi or a car service.

Here's why:=

8:30 touch down. Is that flight usually on time? You can lose one or two hours on most routes quite easily if the weather gods are against you or if your plane has what is euphemistically called a "mechanical".

But if it is on time plan on 90 minutes to taxi, deplane, make the long walk (I've never measured it but it must be approaching a mile), queue up, immigration, baggage collection, and - finally - through the frosted glass into arrivals. At least 15 minutes to get to a car or coach, even if a little (wo)man is waiting for you (no toilet or food stops), and onto the M4, M25 and M23. At least 90 minutes, could easily be 2 hours on the road.

OK, check the watch. What time is it? 11:45 or 12:00? Too late to check in. Sorry.

That was with an on-time plane, the usual 90 minutes at Heathrow, an immediate welcome by a driver and a reasonable walk to the car park and getting the car out, and no significant traffic on the M4 or M25.

Maybe you will taxi right to the gate with no delays, the plane will be a little early because you got a tail wind, you are number 1 off the plane, you do an OJ Simpson though the long walk, first to immigration, no other flights landed at the same time, no luggage, and your driver parked on a double yellow line right outside the door, and you can do the speed limit or a little above all the way.

Maybe you can claw back an hour that way.....

If I were in your shoes I wouldn't try it.