What the hell?!?! 
I have recently realised that most people don't realise that small companies RECLAIM from the Government (HMRC) any statutory maternity payments which they make.
So the 90% of salary for 6 weeks and the ~£123/week for the remaining 33 weeks.
In fact they reclaim 104.5% of it. This covers any employer's national insurance payments so they have NO £ going out on SMP.
(Larger companies claim back 92%),
See here.
They have to pay to recruit a new employee - which they would have to do if the pregnant employee was sacked/left.
They have to train the new employee - again which they would have to do if the pregnant employee was sacked/left.
They put the replacement employee on a temp contract so no liability incurred in terminating them when the woman returns from maternity leave.
If flexible working does not suit their business and they can prove this then they don't have to offer it.
If small companies weren't giving SMP then would maternity allowance be qualified for?
What will stop companies artificially organising to have subsidiaries of less than 10 people to get round the legislation?