I would not recommend it as a place of prayer, certainly not for shy persons or those in distress. No doubt strong and secure Christians will insist on their right to pray to God without payment and will simply ignore the money-changers, but why should people have to go through this hassle to prove their credentials? It seems a kind of ecclesiastical equivalent of passport control.
I would not recommend people who wish to pray in church to go to St Paul's at all. Westminster Roman Catholic Cathedral is more prayerful; it is taken for granted that this is what the cathedral is for, and nobody will give you funny looks when you pray there. There is also St Vedast Church just across from St Paul's, which is a very prayerful space.
The Rev KENNETH LEECH
St Botolph's Church, Aldgate