If you are thinking about doing it now you should be OK because the weather forecast is for a bit cooler and a bit wetter weather . Even so water new plants thoroughly and regularly . But any time now up until the end of autumn for most shrubs and perennials. The worst time is during a really hot spell in mid-summer.

After these 2 harder than normal winters i'd say get things in the ground before the end of summer.

From experience a lot of gardeners have found that planting their shrubs and perennials AFTER then means losses as the plants hadn't been able to get their roots as established as possible

Recommended planting time for most things is actually autumn, with odd exceptions best in Spring (ceanothus, garrya, cistus for instance). Last year was unfortunate though - anyone who planted things in October, which is normally perfectly reasonable and recommended, may well have lost them because November arived and we descended almost instantly into arctic weather, snow and ice. This is very rare though, so I'd plant up to late October here in London.