No they don't have to provide single sex swimming but given that they do, or at least they say they do, they need to ensure it is actually single sex. Sex Matters' letter goes into some detail about the setup for changing and showering at the Ladies' Pond:
6.1.6. The Ladies’ Pond changing hut contains open communal showers (with no shower cubicles) and an open changing area. There are four closed changing cubicles which have gaps at the top and bottom and open onto the changing area. There is one shower with a shower curtain which is not full length.
6.1.7. The open showers and open changing area involve full nudity. There is no place to hang clothes in the curtained shower area so women enter and leave the showers naked.
6.1.8. As well as the indoor showers and changing area, which is quite small, there is an outdoor shower and women change outside in an area adjacent to the changing hut and on the grass slope.
6.1.9. There are only female toilets, and they are the kind with gaps above and below the cubicle doors.
6.1.10. In summary, the facilities are designed and advertised for single-sex use and nudity is common.
The letter also describes how women often sunbathe topless in the upper meadow.
The provision of facilities like these, while telling women they are single sex when they are not, absolutely violates women's dignity and creates an intimidating, hostile, degrading, humiliating or offensive environment for them.