Planning a trip to Bangladesh is actually not a "beginners" job. The person doing this needs to be pretty proficient as we face here about all that can influence a trip planning. The number of countries crossed is numerous as well what doesn't make it easier at all.
What is normally done, when such a trip is set up, is to make sure there is a representative on each side of the trip. This means an handling agent in London and one in Dhaka. Then you need to find out how to get overflight permits for all those countries involved and you will probably need a sponsor for those places as well. As long as the flight goes trough Europe it's pretty simple, as Eurocontrol is the head agency you are going to deal with. They will also invoice you for ATC and airways. Those countries outside Eurocontrols airspace will probably ask for a permission, which is strongly dependable from the payment of the fees. If you don't want to deal with a local agent in those places, it gets real complicated, as the countries then normally ask "payment in advance". But this is not such a problem, as airlines have their contacts and for Private operators are agencies doing such stuff all day long.
Then you need t figure out, which route would probably be best, the day you want to fly. The closer it comes to the flight, the better to plan as the wind and weather data's get more and more accurate, the closer it comes to the flight. The most probable route you will need to ask all your permission's, as you start to know the Entry- and Exitpoints of the countries. It's also about time you will realize if and how many stops will be required for the flight.
Once you got the bureaucracy together, the planning and calculation of the flight itself isn't that much work, as the computer tool will do it. Good tools know the availability of airways, in others you need to be careful not to plan sure closed sectors or one way routes. If all that is together, the flightplans can be filed trough any local AIS in the world. If your information is correct, the flightplans will be accepted, if not they reject it and the process will restart.
You see now, why I asked you what exactly you want to know: A flight from England to France is a piece of cake. There is no permission needed, all you do is figure out the correct ATC route from A to B and file it. Just make sure, that the port of exit and entry of the respective counties are international airports. If they are not, you will need to fly to an international airport first, clear customs and continue. This is normally not the case in airline operation but occurs quite frequent in private- and corporate flying. Sure, if you fly within the Schengen airspace, this is not needed, as you are in the legal circle.
Some countries ask special coverages from insurances as well, specially 3rd party coverage. If you want to fly into such areas, the operator probably need to call his insurance to make sure they cover all. Another story are those countries, declared as "unsafe" by the insurance. You will need an additional coverage to go there, otherwise they won't pay one cent if the plane gets lost. Places as Iraq are actually on this list....
Finally the world is full of exceptions. For example France: They don't allow the commander of an aircraft in commercial use to be more than 60 years of age. I remember a Crossair RJ100 being ramp-checked in Nice with a Captain, being older. They grounded the plane and Crossair had to send a younger Capatin to Nice to fly the plane home.
You see, the world is full of surprises. I haven't planed a trip to Bagladesh yet. I have flown into Thailand, Macao, Hong Kong and China several times and the planning part was quite demanding, although having good support from superior agencies. The details you find in each countries AIP (Aeronautical Information Publication) that can be accessed in many cases over the internet as well. Just make sure, to check the NOTAM's (Notice to Airman). The NOTAM's the publication of the deviations from the AIP. If for example a frequency has changed, it's published in the NOTAM's as a firebrigade not available etc.
I hope, I could give you some ideas. Have a good trip to Dhaka,