The 158 Squadron Memorial, Lissett

So much more tasteful than the English Electric Lightning on a scrap heap I used to see on the east side of the M1!

Collingwood Corner between Pimperne and Tarrant Hinton north of Blandford on the A354 commemorating the Collingwood Batallion from the First World War

You could add in the Stretton War Memorial which is on that A45 roundabout just after the M45. Always wondered how busy the traffic gets there on Remembrance Day and if they stop it all?

Following the A45 theme, there is the cyclists memorial in Meriden.

The Gloster E.28/39 on the roundabout near M1 J20, memorial to Sir Frank Whittle.

Giant chicken on the A24/A25 rbt at Dorking
Concrete elephant on the A30 in Camberley
Silbury Hill, A4

A temporary Queen on the A303 near Sparkford, eastbound side. The arm waves.

Wow, roadside monuments! Probably my favourite conversation point!
To keep this short I will only say ones that have not yet been mentioned.
The 'Peas' bridge on the M25
The tower of light on the M5 near weston-super-mare
The wicker deer at Newtown on the A303
The wall with loads of gnomes on the A30 in cornwall
The wicker sculpture at the exeter inn on the A369
The orange elephant on the A38 at Kennford, abstract sculpture at bristol airport and the wooden dog also on the A38
The various large sculptures of deer etc on the A35 (a bit like some of those that have already been mentioned)
The metal swan, family of bears and the telephone box with a clock attached to it on the A3052 in Devon
The animal topiary on the A377