There are American Robins in UK... but they are not common.
The swallow nests I have seen are more mud and less grass/twigs. Yours must be different in construction.
Swallows here pick much more precarious locations for their nests, often hanging on nothing..but attached to walls with the mud.
Your nest is on a horizontal surface which here is a definite characteristic of robins --who also hang around people with some comfort.
I've drawn many nests during my life.(I also have collected some). They are all very interesting.
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All truths are easy to understand once they are discovered; the point is to discover them.-- Galileo Galilei
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Weezie looking at petunias 8.25.2017
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