LoulaBella – it's really finding a ride that's pitched for what you want. It's a great way of being shown around trails by local riders.
For example, on Sunday there's a Mendips ride and there will be a good turnout for that. Yesterday was Wiltshire, last week was Basingstoke (!), there's also been the Quantocks, Exmoor, Purbecks, another Wiltshire ride, Swinley Forest, Chilterns etc etc!!
The organisor will normally state the distance, type of terrain etc and will be happy to answer questions on suitability. Nobody will get left behind.
I would also agree with offthebrakes recommendation – the Tunnel Hill Trolls have a ladies section and they are a really nice bunch of girls. They sometimes ride in other parts of the South too.
This forum can seem quite a terrifying place but I have met so many nice people, both male and female, and the riding has been brilliant 8)
Hope this helps!