It turns out that pregnant women who are on Statutory Sick Pay when they are self-isolate can miss out on Statutory Maternity Pay. No mention of this in the PM's grand announcements about pregnant women. And no guidance telling employers that they can't dismiss a pregnant woman who needs time off for self-isolation. Either the Government doesn't understand the financial pressures of having a new baby, or they don't care.
Eligibility for Statutory Maternity Pay is based on your earnings during the calculation period, which is roughly weeks 18-26 of your pregnancy. You need to have average earnings of £118 per week during this period, and Statutory Sick Pay is well below this. If you don't meet the threshhold, then you don't get Statutory Maternity Pay. You can probably get Maternity Allowance, but it is a lower rate of pay for the first six weeks and you can miss out on other benefits.