Technically the restriction=no_u_turn should only be used on signed no u turns, and not as an arbitrary way to place restrictions on the routing, but it is common practice it is widely used as the latter and seems mostly accepted to do this by the community.

So I would say in this case to prevent routers trying to u-turn from Ellerton Drive back onto Ellerton Drive where the dual carriageway merges into single it is acceptable to use a no_u_turn relation here, even though it may not be an intersection and even though there is no signage.