Seen by the river Thames this bird - according to the RSPB Handbook of British Birds - seems to be a whitethroat. One of nearly 20 different types of warbler that can be seen in the UK in spring and summer:
It looks very similar to the lesser whitethroat, but the rear plumage looks darker:
It is a summer visitor, choosing to breed in the UK and then in autumn migrate to North Africa. Apparently there are an estimated 650,000 breeding pairs in the UK. As always, I'm happy to be corrected if my identification is incorrect.
This wren is almost unmistakable. One of the commonest UK birds with an estimated population of 8,000,000 breeding pairs. For such a small bird it has a surprisingly powerful and distinctive song.
Unlike the whitethroat, wrens do not migrate and can suffer as a result. In a harsh winter they can suffer 80% mortality.