These floats seem to be available in some serious sizes (up to 15g) I don't have any floats bigger than 5g so what sort of size is preferable for the Wye or Severn and would you use the "smaller" 3g floats for Salwarpe or Teme?
Also, any special shotting patterns?
I have experimented with these floats in 'shallow' 3ft ish swims and had problems; in deeper water the floats are far more stable.
In terms of float size, sorry I can't help, as I don't fish the rivers that you mention, these floats were developed for fishing big European rivers and so would be suited to fishing when there is a bit of water pushing though.
With all floats as long as you have it correctly shotted, the weight that it carries can be 'discounted', you can use 15g without issue, getting the bait down to where you're fishing and maintaining control is what you're trying to achieve, if it suits, you can also 'overshot' the float when holding it back creating a very sensitive rig.
http://http://youtu.be/v0epiuooOWQ Its in a foreign language but it looks like he's on the Itchen.
If you are thinking about buyng one or two Cralusso floats, buy a couple in the sizes that you are familiar with and fish with them, as long as you're not fighting the conditions you should have success.