I am not aware of any advice or instruction to stay indoors, no ash or dust can be seen here, it is a bright sunny morning with a clear blue sky.
Life carries on as normal, except of course for those wishing to fly, or otherwise reliant on the aviation industry.

One should of course be prepared with supplies and plans etc. for sheltering in place, in case of pandemics, radiation release, escape of toxic chemicals, or natural disasters.
There does not appear however to be any need for sheltering in place at present in the UK.