Split into 2 separate ltd companies.
One commercial window cleaning company that is VAT registered, one purely domestic non vat registered and below the threshold? One is business to business and one is business to customer??
As mentioned - this would fall under artificial separation. There is no practical reason why the businesses need to be separate - you are cleaning windows ... end of.
My wife is self employed as a part time cleaner and a little bit of bookkeeping she also does my books and tax returns for which she invoices me. I am self employed too. Our joint turnover does not reach the VAT threshold but if we both expanded our operations it would be under accountants advice whether we could run separate cleaning and window cleaning businesses.
The last time I asked the question I got a common sense answer along the lines of "If the taxman asked why you run your window cleaning business and your wife runs her domestic house cleaning business separately, what would the answer be?"
This is because for keeping running costs down you would use any vans to both your best advantage, the same with office space, computers, mobiles etc., answering the phone for each other ... and on ... and on. The only reason to separate would be tax saving that would compensate for the above. So a big no ... no.
This could be OK if you had two obviously separate businesses such as (e.g) your wife running a crafting business and you running a window cleaning business. But even then there are strict rules such as shared computers, vehicles, addresses that raise concern about whether you are doing it to avoid tax.