I have seen these trees across London in different gardens
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Cercis has pea flowers in trusses erupting from the main limbs/branches which this has. Leaves orbicular with cordate base
... of the photographs.
We now have the question whether the other specimens the original poster has seen are all Cercis, or whether they are a mixture.
Does anyone know how common Cercis is in London?
I cannot say for London, but it has been used as a street tree in Oxford or Cambridge, unable to recall which or whether it's both.
Its a neat tree. Here in Leicester it is to be found in some of the early 1900s properties about he city.
The leaves don't fit Syringa vulgaris.
Cercis siliquastrum is commonly planted
on the contient, I can confirm what GrizzeldBedger says, there is at least one planted in Oxford centre, but I am not sure about London. It is native to SE Europe and further East - if I remmeber well.
Lat/Lng: 51.5164, -0.1008
OS grid ref: TQ318814