Putting aside all the little legal caveats that are based on UK law and will be same for all UK firms, our maternity policy basically states:
* "Reasonable" paid time off for antenatal activities and appointments, plus
* 8 weeks "ordinary" maternity leave at full pay
* 18 more weeks "ordinary" maternity leave at the SMP (Statutory Maternity Pay) standard set by the government
* 26 more weeks of "additional" maternity leave which is unpaid
* Benefits continue as normal throughout
This policy is for a medium-size international consultancy (ca. 2,000 UK staff)
Note that some staff have more generous policies because they were part of firms that were bought. Some senior consultants and principals have negotiated more favourable conditions (26 weeks full pay and retaining bonus eligibility).
I recommend that you cost out the policy decisions both for your present firm size and taking into account potential changes a few years down the line. When drawing up the policy, invest in getting it done by an employment law professional to make sure it is watertight (for both the firm's and the employees' benefit).