Leaves make excellent mulch; the only downside is that too many/too large leaves can smother perennials and/or cause crown rot in some species. Also, the species of tree that the leaves come from makes a difference - trees from the Populus genus (aspens, cottonwoods, poplars) tend to become leathery and not break down very easily, while those of Juglans nigra (black walnut) can be slightly poisonous to some perennials and grasses.
Another issue with leaves is that they tend to blow away from the garden, or within the garden, when they dry. This can leave some areas of your garden knee-deep in mulch while others are bare.
Your beech leaves should be fine as-is, because they tend to not be particularly large, but sycamore leaves can be quite wide and more prone to smothering things. A good strategy if you have a power lawnmower handy is to shred a pile of dry, raked leaves by repeatedly running the lawnmower over the pile. When you put the shredded leaves in the garden, they won't blow away, and shredding them also helps them to decompose faster. Sometimes, the shredded leaves will completely break down over the course of the winter, meaning that you'll need to apply another mulch for the growing season.
I've had good luck with an initial layer of woodchips on top of which I put the shredded leaves (oak and basswood [lime in the UK] in my case). When the leaves decompose, they leave the woodchips behind for the summer.
As for encouraging weeds, as long as they're covering the soil the leaves will inhibit germination; once they're gone and the soil is "open", then weed seeds will germinate and be happy to take advantage of the nutrients left behind by the now-vanished leaves. This is why another mulch for the growing season is a good idea - either something like woodchips, cocoa bean hulls, rice hulls, pine needles, etc. - whatever is most readily available in your area.