Hi Sue,
What a problem you have!
When was he adopted?
Firstly have you looked at the 1911 Census for Arthur to check out the Household?
It looks to me he is down as Arthur Cecil Upright born c 1904 and is in Edmonton reg district, Middlesex;
Other Uprights in Edmonton in 1911 are;
Mary Anne, 65
Mrs Dorothy, 32
Ernest William, 25
Catherine Marjorie, 25
ARTHUR CECIL, 7
Albert Edgar, 3
Douglas Gordon, 2mths#
Could Dorothy be Dora?
There is a Death Reg for a EDMUND Luther Upright, September 1909, Edmonton, 3a 192, Age 54
Also a Douglas G Upright aged 0 in September 1911, Edmonton, 3a 264#
You need to check out the 1911 to see what his relationship is to the head of Household. And which of these Uprights are living together. (they could be 2 separate households)
Let us know the outcome please.
Trish

EDIT Ernest and Catherine could be a married couple as there is a Marriage March 1911 with a Ernest W Upright and a Catherine M Mould on the same page.
Yes, it is possible Edmund used a stage name as I cannot find him in previous census 1901/1891 but there are possibilities further back.
EDIT With a name like Cameron, Dora and Edmund may have married in Scotland or anywhere if they were both in the Acting Profession, so you would not find it on FreeBMD.