There a st albans mum group.. there was an event like this on in October but weren't much. It wasn't exchange just went to look. I think it's more giving away clothes. If you have time why don't you set something like this up?
At the methodist church in the town near the maltings car park, they sometimes do event days (via the family centre) and they've done clothes swaps there before. May be worth looking out for one if those or asking the family centre/health visitor about it.
The clothing loop too is good in st Albans.
I’m the admin of the women and children’s clothing loop. Basically there are bags of clothes, you get on, take out what you want and put in your unwanted clothes. If you want to join I can send you the link x
I don't know of anything like that but we've got loads of clothes from this Facebook group: https://www.facebook.com/share/g/19eTguMsjE/?mibextid=K35XfP