It is a mechanical device.
It is unique.
It has a certain charm, but it is not classed as Art.
It is not a building, though it was built.

Its purpose is not entertainment.
It is located in the UK.
It was built for transportation purposes.
I'm not sure if it's regarded as old-fashioned.
It's not a train.

It is a bridge.
It is an A-road.

It is not the Iron Bridge in Coalbrookdale.
It is not in London.

It does not cross the river Forth.
It is not on the A5.
The name does supply a vegetable connotation. But it's such a punnish and abstruse one, that to be honest I can't recommend this line of investigation.
It is not the Prince of Wales' bridge on the M4.
It is not the Menai bridge.
If you don't want a hint, read no further.

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Remember that this is a mechanical device.