Is this reasonable? 8am international arrival heathrow 10:30 am transfer bus 3:30pm flight out LTN

That's cutting it fine. If the bus is directly to Luton Airport (not the town) 1300 would be in time for your flight, but with the potential for flight delays across the Atlantic, immigration delays at Heathrow, and traffic between the two airports, I would have a night in London instead.

No one here knows. Keep in mind that this is all contingent on your flight from the states and whether it will arrive on time. If it is on time, then you should be able to catch the bus to Luton and get to your next flight on time.

That said, there is a National Express bus that leaves every 20 minutes from Heathrow T2-3 to Luton and is non-stop (takes a little over an hour), so I'm not sure why you are saying that there are only two buses that work for you.

ETA: Here is a link to National Express. Search for trips on the date you need and see what pops up. https://www.nationalexpress.com/en

Is there a reason ( other than price) that you are not flying out of Heathrow to Budapest? It appears that London Luton flights to Budapest are on Wizz Air. Wizz Air was named the worst short-haul airline by the British public last year in an industry wide survey. It has a record of canceled and delayed flights.
The conclusion of Rory Boland, the travel editor of the consumer group publication “Which?” that conducted the survey was: “Avoid Wizzair as its poor record on customer care means you’re likely be left high and dry in the event your flight is delayed or canceled.”
If you look online there is an avalanche of bad reviews and even videos ripping Wizzair.
www.Skyscanner.com shows your flight options from London to Budapest.

Something that I failed to mention. I am traveling on a Saturday, if that changes anything.

Based on your information about Wizz air maybe the Ryanair flight here would be better and yes cost is a factor.

There is another flight at 16:35 out of STN I could take instead.
The bus there leaves at 11:00 and arrives at 12:25.

Alternate, probably too close. The bus leaves at 12:30 and arrives at 13:55.

Is there a fast way to get to STN that is not National BUS that I could use instead as there is no bus for over 90 minutes between 11 and 12:30. Maybe there is a fast way, other than cab or Uber?

The problem is if I don't make it this day. I won't be going.

The bus schedules are based on the National BUS website. If this information is incorrect or there is another way then please let me know.

Is this on one ticket? Or separate tickets?

Seperate tickets and I do know the risks. I cannot afford one ticket as the cost is extreme. I will be getting travel insurance for delayed/ canceled flights.

The buses run every 20 minutes from Victoria to Luton airport, but I think the OP has his frequency and timings right from Heathrow.

For Mark- get the Elizabeth Line or the Piccadilly Line (the Elizabeth is only about 5 or 10 minutes faster and has a change at Faringdon) to St Pancras International then an East Midlands Train to Luton Airport Parkway, then the DART shuttle train Railway station to airport terminal. Far more frequent, takes 1:45, say 2 hours to allow for connections.

Yes if there is a later flight, take that. Better to hang around and have lunch at St Pancras than be racing round.

While I appreciate wanting to make the best use of your time, there DOES look to be an evening flight on Ryan from Stansted and on Wizz from Luton. One of those might be wiser.

The 100E airport bus AND taxis will be available upon arrival in Budapest even with a late arrival. The bus might be every 30-40 min instead of every 10-15 min. is all. Makes no difference for taxis.

Thanks, With those directions you must have missed that the later flight is at STN and not LTN. Good information though if I do need to do it.
According to Google, I would take the Elizabeth line towards Abbey Wood and then the Stansted Express towards Stansted Airport.

Google Maps says I could leave Heathrow at 12:30 and still be at STN by 14:00 for a 16:30 flight. Is 2.5 hours enough for Ryanair out of STN?
I did look into flights out of Heathrow, and they are all very late and expensive.

I don't like the idea of doing it this way, but this is the only way I can get to Budapest with my time.

For those of you that fly would you risk this? What would you rate the risk level of a scheduled 8 am arrival and a scheduled 16:35 take off at a different airport?

Thanks,
Mark

Sorry, yes STN is Elizabeth Line to Liverpool Street, then Stansted Express. Leaving at 1230 I think 2pm is a mite optimistic.

Officially it is 1225 arrives 1411 and 1236 a 1431.

For the 1225 to arrive at 1357 you need a 8 minute connection at Liverpool Street between lines. If dead on time I could probably do that, because I would be on auto-pilot and moving at pace. I'm not at all sure that you could.

Nat Ex have buses to STN at 1105, arrive 1225; 1235 arrive 1405 and 1305 arrive 1425. (also 0915 arrive 1035). Just board and relax- far less stress IMO. The 1235 is 1 stop, the 1305 is no stops.

Before committing track your TA flight's performance for, say, a week. Then you can make a more informed judgement.

Assuming a good on time performance of the TA, if that was my best option it's good enough IMO.

You know, we all know, that it's a gamble but the odds, I contend, are in your favour. If TTM has found an even later Wizz flight that would be even better.

Play it by ear. Bus and train can both be booked on the spot. Take the most expeditious mode on the day. Being a Saturday weekend engineering work may disrupt either train line.

Please pay attention to the last sentence in isn31c’s post. Weekends are frequently when needed maintenance work is done.

That sounds risky in my opinion. Is there a special reason you are flying to Heathrow? If you can find a flight to e.g. Vienna or Munich you would have a lot more options if you miss your flight to Budapest.

Thanks, isn31c, for the advice about the on-time performance tracking. I didn't know that was a thing.
It's hard to judge right now as the weather is most likely the biggest factor, that will not be nearly the challenge when I want to go in late May, but it does at least give me an idea.
For the last two months, the flight had 51 flights on time or early, with 3 slightly late and 4 very late; strange is it had 7 diverted, with no explanation. I could only imagine bad weather at Heathrow. For winter months I don't see this as bad. It also typically arrives 30-45 minutes early and sometimes up to an hour early.
There was a chart that showed a year with no numbers, and in the Spring and summer, it was almost 100% on time. I will keep looking for another month or so and plan on taking the bus as you have suggested and avoiding the train if possible.

As far as flying to another destination closer it is much, much more expensive and makes this trip impossible.

Another thing to factor into the equation is what time the airline closes the door.

Some airlines close boarding 30 minutes before the scheduled flight time.

Another thing that can add time at both ends is checked bags. Your post is silent about this.

How much time you really have is probably less than it appears based on scheduled arrival and departure times and published transit times between airports.

There is a bus at T2/3 at 10:30 am that will get me to LTN at 11:20 am

Fastest journey time 50 mins

(from the National Express site. Your bus may not achieve "fastest" status.)

Wizzair gets bad reviews from people who want more than what they paid for. Otherwise the worst that can be said is they are unimpressive. But when I want to get from A to B l don't need to be impressed.