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Amsterdam, Paris (I think), Somme Poppy marathon, Nice-Cannes, Marathon du Medoc.
I guess they have more room in France!
I wouldn't fancy a multi lap mara either.
Greensand is an out and back so not technically a lap.
Farnham Pilgtim is an out and back over part of the course.
Venice is one way.
Florence, Rome, Parma, Milan and Turin are all one way (I think)
Berlin is a single lap
Moray Marathon in September
Windermere - once round the lake
Isn't there a new one planned early in the year down Blackpool way? I thought that was an out&back route
Lochaber - out&back?
I believe Edinburgh is point to point
Robin Hood marathon in Nottingham would seem to fit the bill, you run a half marathon with the half marathon runners and then the second half is on a different route altogether, so not a multiple lap race