Am I correct in thinking that this holds true in all circumstances
IF one is not trying to commit fraud.
In the UK it is legal to call yourself anything you want as long as it is not for financial gain. You do not need a deed poll as others have indicated. You just adopt the new name and inform everyone. This right is enshrined in Case Law from the 18th century, so dont be bullied into anything else!
SO on the census form I could call myself Boris Johnson or whatever... obviously if I use a less obvious name which is an alternative to my own it is less likely to attract unwanted attention.
Obviously I could decide to go back to using my old name quite quickly so that there would be no point it me informing anyone.