John wrote:
...
A roll cloud was observed in Hampshire this morning , ~7.30 BST,

Saw the cloud in Shrewton, Wiltshire at 0815BST. It approached from the
south east quite slowly. When overhead, it went very dark and there was a
line of low stratus underneath. This was about 0835BST. No rain. Brightened
up once it had passed and then started raining about 0845BST. Thunder soon
heard with one very close strike about 0920BST which cut the electricity for
about 10 secs.
Timing quite accurate as I take my son to school at 0845.

Thanks John,
this suggests the cloud was doing around 40mph (!), in agreement with
the first observers estimate.

Any further obs from anybody wld very useful,
David