Why are roundabout entry lanes not better marked

[GT6k]

I regulalrly cross a couple of roundabouts where there are multiple entry lanes but all the exits are single lane. This is compounded by layouts without a clear straight on direction and particulaly where straight on is slightly right. It would seem easy to mark one of the lanes as right or left to reduce confusion.

A good example is here, which lane do I choose to carry on straight ahead (actually slightly right) on the major road.

https://www.google.co.uk/maps/@51.5778335,-1.73006...

[Munter]

Both lanes. That's why there are 2 lanes off at the 2nd exit and a merge in turn.

It's also why that road is a thick line on the sign, and the other 2 exits are thin.

[GT6k]

Thanks that makes sense, it just seems more risky than marking the enty lanes and having the traffic already merged for the exit. I never know whether the car in the other lane is going to turn off or carry straight on and only finding this out in the 25yds of merge is a bit too late.