Although called 'Californian Lilac' in common parlance, I don't think Ceanothus looks anything at all like lilac. There are several species and cultivars. Most are not very hardy. You would have to make sure you chose a type suitable for your zone and soil. This is Ceanothus thyrsiflorus, not mine but growing just down the road.

{{gwi:196484}}

As flora indicates, this shrub tends to be rather specific to its growing requirements. The evergreen forms - the ones commonly referred to as 'wild' or 'California lilac' - are not considered hardy below USDA zone 8 and tend to be somewhat short-lived unless in full sun, extremely well-drained soil and are neglected as to irrigation and fertilization once established. Can't speak to the UK, but these shrubs tend to do best in the US in a very mild, coastal maritime location. IOW, one doesn't encounter them away from the Pacific Coast.

flora_uk SW UK 8/9
I assume it is the one on the right. It is so beautiful!

gardengal48 PNW zone 8
Thank you for your explanation. So, I guess I should not consider it because I could not provide the mild weather or the dry soil ...

Oddly, they are grown reasonably successfully (and frequently) in England. I saw them all over Kent, sometimes espaliered, along with Fremontodendron, in mixed borders! All received if not copious, at least meaningful, summer water. A California native plant expert told me later that because the temperatures in England were much cooler in summer than California, the natives such as Ceanothus and Fremontodendron were not susceptible to the fungal diseases that quickly kill them here if they get appreciable summer water.