Ah ... mixture. Close, but no cigar!
A ticket from Helsinki needs to be calculated in euros, regardless of country of sale. If tickets from Finland are purchased outside of Finland then the final euro price is converted to the currency of country of sale via the IATA rate of exchange (IROE). (ie euro price converted to NUCs (see below) and then converted to GBP)
In a complicated journey involving many fare components each of those 'local currency' components are converted to NUC's (Neutral Units of Construction) at the IROE; summed and then converted to the currency of the country of sale at the IROE.
It sounds complicated but it isn't (the work is does by computers) but is easier than adding pounds to euros to rubles to Thai baht to AU Dollars to Canuck Bucks etc. and then converting everything back to pounds.
Back to the original question. I would go to someone who is experienced in RTW journeys (like Travelbag or Trailfinders) and ask them for a price for a specific journey, naming your desired (and optional) stopping points, and desired departure and return dates. RTW calculations are complicated (nature of the beast) and a simple change of itinerary or dates can make a big difference. Before they go to work they want a reasonable assurance that you will buy from them.
Also don't forget that RTW tickets do require you to end your journey in the country of origin (If you're ticketed from Finland you should end your journey there as well) So factor in a RT ticket to/from your point of origin. I think you will find that most RTW prices from Europe are going to be similar in price - tickets from places like Thailand are the ones you have heard about as being really inexpensive!
*A NUC is actually a US Dollar, but IATA wasn't allowed to call it a dollar for political reasons. Some governments (who have to approve IATA Resolutions) would not accept that term. Nobody seems to mind calling a spade something else. (Old joke: What's a camel? A horse designed in IATA)