Indeed, welcome, Yohan!

A few points to consider:

- In addition to cheaper or more expensive base prices depending on the country of origin, the choice of issuing carrier and the original departure point can matter a lot. British Airways, for example, adds heavy surcharges and fees to RTW tickets that they issue; the same ticket issued by another airline might have a surprisingly smaller "bottom line." Once you get into the thicket of fare construction and taxes/fees computation, it all goes into a black box at the issuing airline, and - sadly - these computations are neither transparent nor debatable. The Oneworld online tool is notoriously buggy on this front; many people think the tool has given them a good fare but when they go to put in their credit card information it all goes to hell.

- Starting longhaul flights from places like the UK can attract big departure taxes (in the UK, air passenger duty) - a series of these can bump the final price quite significantly.

- You're probably not in a position to know what your travel patterns will be going forward, so using an RTW trip to amass a quantity of frequent flyer points (miles/Avios etc.) would only be beneficial if you knew how you'd plan to use them. But given you have time to plan and do "what if" thought experiments, why not take the time now to "model" your proposed trip with a couple of major FF schemes. How many Avios and tier points would your itinerary earn if you credited them to BA? How about to Iberia? You can look up the Avios and TP earning tables for both economy (fare code L) and business (D) and see the differential. Note that if you plan to travel after the RTW, the elite status might mean more to you than the redeemable miles/points. Or with BA, your Avios might not get you a lot of free flights, but might enable you to upgrade using points more often than you'd do otherwise.

- Especially for residents of the UK, starting somewhere else not only can help with the base price and taxes/fees/surcharges, it can also allow you to start or end using home as a stopover point. For example, the cheapest origin point for business class tickets in Europe is presently Norway; a DONE4 from Oslo has a base price £684 less than one from the UK. And if your first flight was to, say, Helsinki, you could have Finnair issue the ticket and possibly avoid big BA surcharges. You'd have to pay for "positioning" flights but those costs would be far less than the savings on the RTW ticket itself. (FWIW Norway is almost £200 less than Spain or the Euro zone for an LONE4 and almost £500 less than Spain/Euro for DONE4s.)

- Finally, you could also use your time to see if an RTW ticket makes the most sense in terms of total cost. Look at these FT boards - http://www.flyertalk.com/forum/mileage-run-deals-372/ and http://www.flyertalk.com/forum/premium-fare-deals-740/ where members post alerts on good deals. For example, right now there's a thread where people are posting business class deals to/from Australia for £1500 or so. It's entirely feasible to construct a flight plan that uses cheap business class return fares for longhaul segments but also uses low cost carriers or inexpensive economy fares for shorthaul ones, where the perks of business class - flat beds, lounges, etc. - might not be as important. Most people can survive a couple of hours in an economy seat in Europe or in Asia without lasting damage.

Again, welcome to FT and to our mad little corner of RTW travel!