Are there any other features I could go back and observe to help be sure which Populus it is?
So long as it is tremula, alba or x canescens then the leaf miner on it is still identified correctly (if it were nigra or x canadensis it would be something else).
... Populus canescens or Populus alba. However the foliage of suckers and young plants tends to be atypical - I've been confused by such material of aspen before - otherwise Rachy's rejection of Populus alba on the grounds of the absence of distinct lobing of leaves would stand. I favour Populus alba on the grounds that the indumentum of Populus canescens is less persistent, and it doesn't sucker/seed so readily.
Lat/Lng: 51.540316447216, -0.32921433448792
OS grid ref: TQ159837