I am finally getting round to getting my small back garden into some
sort of shape.

I have even planted something [1] which was rather fun.

My beginners question is - is it actually a good time to order new
plants (e.g. perennials or an Acer) and stick them in the ground now or
should I really wait until next year?

The J Parkers catalogue [2] suggests it is a good time to plant before
dormancy sets in. I'm just a bit dubious as to whether "baby" plants
will root in time and not die over winter - especially in quite heavy
London clay soils.

[1] some grasses and hellebores. Bulbs go in soon providing the soil
doesn't become horrendously wet.
[2] free with gardeners world magazine.
--
Paul C