I don't really bother with substitutes. M&S do "Free from Wheat" cakes, the lemon one is OK but a bit dry & bland. There was a yummy cherry chocolate one but the swines discontinued it.
You can make wonderful mini quiches by using silicone large muffin moulds (John Lewis, or Lakeland sell them) just don't bother with pastry at all, pour your quiche mixture straight in. If you want a bit of crunch you can add a thin base of crushed nuts bound with a bit of butter at the bottom of the moulds.