Some considerations:
- purchase cost of most RTW tickets varies by country, so it isn't the same everywhere in Europe. UK is one of the more expensive origination points. The price is different for each product, so which country is cheapest to start in depends upon which RTW product you are buying, but for example Sweden is a popular starting point.
- most discount fares have maximum stay restrictions, often 30 days and rarely over 90 days, so your JFK-LHF-JFK ticket might be more expensive than you expect if you want to stay for a full year. Using award tickets is a common way to get around this.
- it is fairly easy to break a RTW ticket into multiple segments, and get 3 or 4 vacations out of it. E.g. fly to Asia on the RTW, buy cheap return ticket e.g. BKK-JFK-BKK and continue the RTW a few months later.

Lots more, but this should give you the idea of what to look for.